[runtime] Adopt safer native compiler flags: Wshadow (#24088)

Example warning:

```
macios/runtime/trampolines-invoke.m:212:15: error: declaration shadows a local variable [-Werror,-Wshadow]
  212 |                 const char *type = xamarin_skip_encoding_flags (argType);
      |                             ^
```

References:

* https://developer.apple.com/documentation/xcode/enabling-enhanced-security-for-your-app
* https://releases.llvm.org/8.0.0/tools/clang/docs/ReleaseNotes.html#major-new-features (for -ftrivial-auto-var-init=zero)

Contributes towards #23023.

Author: rolfbjarne

mk/rules.mk

@@ -42,6 +42,7 @@ CFLAGS=\
 	-Wcast-function-type-mismatch \
 	-Wsemicolon-before-method-body \
 	-Wsign-compare \
+	-Wshadow \
 	-g \
 	-I.
 SWIFTFLAGS=-g -emit-library

runtime/monovm-bridge.m

@@ -267,7 +267,6 @@
 bool
 xamarin_is_class_nsstring (MonoClass *cls)
 {
-	MonoClass *nsstring_class = xamarin_get_nsstring_class ();
 	if (nsstring_class == NULL)
 		return false;
 

runtime/runtime.m

@@ -2176,9 +2176,11 @@ -(struct NSObjectData*) xamarinGetNSObjectData;
 	MarshalManagedExceptionMode mode;
 	GCHandle exception_gchandle = INVALID_GCHANDLE;
 
-	GCHandle handle = xamarin_gchandle_new (exception, false);
-	mode = xamarin_on_marshal_managed_exception (handle, &exception_gchandle);
-	xamarin_gchandle_free (handle);
+	{
+		GCHandle handle = xamarin_gchandle_new (exception, false);
+		mode = xamarin_on_marshal_managed_exception (handle, &exception_gchandle);
+		xamarin_gchandle_free (handle);
+	}
 
 	if (exception_gchandle != INVALID_GCHANDLE) {
 		PRINT (PRODUCT ": Got an exception while executing the MarshalManagedException event (this exception will be ignored):");
@@ -2197,7 +2199,7 @@ -(struct NSObjectData*) xamarinGetNSObjectData;
 	switch (mode) {
 #if !defined (CORECLR_RUNTIME) // CoreCLR won't unwind through native frames, so we'll have to abort (in the default case statement)
 	case MarshalManagedExceptionModeDisable:
-	case MarshalManagedExceptionModeUnwindNativeCode:
+	case MarshalManagedExceptionModeUnwindNativeCode: {
 		//
 		// We want to maintain the original stack trace of the exception, but unfortunately
 		// calling mono_raise_exception directly with the original exception will overwrite
@@ -2210,7 +2212,7 @@ -(struct NSObjectData*) xamarinGetNSObjectData;
 		// to throw an exception that contains the original stack trace.
 		//
 
-		handle = xamarin_gchandle_new (exception, false);
+		GCHandle handle = xamarin_gchandle_new (exception, false);
 		xamarin_rethrow_managed_exception (handle, &exception_gchandle);
 		xamarin_gchandle_free (handle);
 
@@ -2231,8 +2233,9 @@ -(struct NSObjectData*) xamarinGetNSObjectData;
 		xamarin_handling_unhandled_exceptions = 0;
 
 		mono_raise_exception ((MonoException *) exception);
-#endif
 		break;
+	}
+#endif
 	case MarshalManagedExceptionModeThrowObjectiveCException: {
 		GCHandle handle = xamarin_gchandle_new (exception, false);
 		NSException *ns_exc = xamarin_unwrap_ns_exception (handle, &exception_gchandle);
@@ -2295,13 +2298,14 @@ -(struct NSObjectData*) xamarinGetNSObjectData;
 	case MarshalManagedExceptionModeUnwindNativeCode:
 #endif
 	case MarshalManagedExceptionModeAbort:
-	default:
-		handle = xamarin_gchandle_new (exception, false);
+	default: {
+		GCHandle handle = xamarin_gchandle_new (exception, false);
 		const char *msg = [xamarin_print_all_exceptions (handle) UTF8String];
 		xamarin_gchandle_free (handle);
 		xamarin_assertion_message ("Aborting due to trying to marshal managed exception:\n%s\n", msg);
 		break;
 	}
+	}
 }
 
 void

runtime/trampolines-invoke.m

@@ -79,13 +79,13 @@
 }
 
 void
-xamarin_invoke_trampoline (enum TrampolineType type, id self, SEL sel, iterator_func iterator, marshal_return_value_func marshal_return_value, void *context)
+xamarin_invoke_trampoline (enum TrampolineType trampoline_type, id self, SEL sel, iterator_func iterator, marshal_return_value_func marshal_return_value, void *context)
 {
 	MonoObject *exception = NULL;
 	MonoObject **exception_ptr = xamarin_is_managed_exception_marshaling_disabled () ? NULL : &exception;
 	GCHandle exception_gchandle = INVALID_GCHANDLE;
-	bool is_static = (type & Tramp_Static) == Tramp_Static;
-	bool is_ctor = type == Tramp_Ctor;
+	bool is_static = (trampoline_type & Tramp_Static) == Tramp_Static;
+	bool is_ctor = trampoline_type == Tramp_Ctor;
 	const char *ret_type = NULL;
 	MonoType *sig_ret_type = NULL;
 
@@ -148,8 +148,6 @@
 	void *iter = NULL;
 	gboolean needs_writeback = FALSE; // determines if there are any ref/out parameters.
 	MonoType *p;
-	int ofs;
-	unsigned long i;
 	unsigned long mofs = 0;
 
 	unsigned long desc_arg_count = num_arg + 2; /* 1 for the return value + 1 if this is a category instance method */
@@ -207,11 +205,11 @@
 	if (exception_gchandle != INVALID_GCHANDLE)
 		goto exception_handling;
 
-	for (i = 0, ofs = 0; i < num_arg; i++) {
+	for (unsigned long i = 0, ofs = 0; i < num_arg; i++) {
 		const char *argType = [sig getArgumentTypeAtIndex: (i+2)];
 		const char *type = xamarin_skip_encoding_flags (argType);
 		unsigned long size = xamarin_objc_type_size (type);
-		int frameofs = ofs;
+		unsigned long frameofs = ofs;
 		p = mono_signature_get_params (msig, &iter);
 		ADD_TO_MONOOBJECT_RELEASE_LIST (p);
 		LOGZ (" argument %i's type: %s (argType: %s)\n", (int) i + 1, type, argType);
@@ -633,7 +631,7 @@
 		iterator (IteratorStart, context, NULL, 0, NULL, &exception_gchandle); // start
 		if (exception_gchandle != INVALID_GCHANDLE)
 			goto exception_handling;
-		for (i = 0; i < num_arg; i++) {
+		for (unsigned long i = 0; i < num_arg; i++) {
 			const char *type = [sig getArgumentTypeAtIndex: (i+2)];
 
 			type = xamarin_skip_encoding_flags (type);