[runtime] Adopt safer native compiler flags: Wmissing-field-initializers (#24071)

Example warning:

```
In file included from macios/appex-deduplicate-frameworks/macios/runtime/runtime.m:98:
macios/runtime/delegates.inc:189:41: error: missing field 'register_assembly' initializer [-Werror,-Wmissing-field-initializers]
  189 | static struct Delegates delegates = { 0 };
      |                                         ^
```

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

@@ -37,6 +37,7 @@ CFLAGS=\
 	-fdiagnostics-absolute-paths \
 	-Wno-objc-protocol-property-synthesis \
 	-Wignored-qualifiers \
+	-Wmissing-field-initializers \
 	-g \
 	-I.
 SWIFTFLAGS=-g -emit-library

runtime/coreclr-bridge.m

@@ -425,7 +425,7 @@
 		return NULL;
 	}
 
-	struct stat stat_buf = { 0 };
+	struct stat stat_buf = { };
 	if (fstat (fd, &stat_buf) == -1) {
 		LOG (PRODUCT ": Could not stat the runtime config file '%s' in the app bundle: %s\n", path, strerror (errno));
 		close (fd);

runtime/delegates.inc.t4

@@ -25,7 +25,7 @@ struct Delegates {
 <# } #>
 };
 
-static struct Delegates delegates = { 0 };
+static struct Delegates delegates = { };
 
 static GCHandle
 create_linked_away_exception (const char *func)

runtime/runtime.m

@@ -186,7 +186,7 @@
 	(void *) &xamarin_get_nsobject_data_trampoline,
 };
 
-static struct InitializationOptions options = { 0 };
+static struct InitializationOptions options = { };
 
 #if !defined (CORECLR_RUNTIME)
 void

runtime/trampolines.m

@@ -905,7 +905,7 @@
 
 	pthread_mutex_lock (&gchandle_hash_lock);
 	if (gchandle_hash == NULL) {
-		CFDictionaryValueCallBacks value_callbacks = { 0 };
+		CFDictionaryValueCallBacks value_callbacks = { };
 		value_callbacks.release = release_gchandle_dictionary_entry;
 		gchandle_hash = CFDictionaryCreateMutable (kCFAllocatorDefault, 0, NULL, &value_callbacks);
 	}
@@ -1411,7 +1411,7 @@
 		element_class = e_class;
 	}
 
-	struct conversion_data data = { 0 };
+	struct conversion_data data = { };
 	data.domain = mono_domain_get ();
 	data.element_class = element_class;
 	data.element_type = mono_class_get_type (data.element_class);
@@ -1437,7 +1437,7 @@
 		element_class = e_class;
 	}
 
-	struct conversion_data data = { 0 };
+	struct conversion_data data = { };
 	data.domain = mono_domain_get ();
 	data.element_class = element_class;
 	data.element_type = mono_class_get_type (data.element_class);
@@ -1463,7 +1463,7 @@
 		element_class = e_class;
 	}
 
-	struct conversion_data data = { 0 };
+	struct conversion_data data = { };
 	data.element_class = element_class;
 	data.element_type = mono_class_get_type (data.element_class);
 	data.iface_token_ref = iface_token_ref;

tools/common/StaticRegistrar.cs

@@ -3107,7 +3107,7 @@ void Specialize (AutoIndentStringBuilder sb, out string initialization_method)
 				sb.WriteLine ();
 			}
 
-			map.AppendLine ("{ NULL, 0 },");
+			map.AppendLine ("{ NULL, 0, 0 },");
 			map.AppendLine ("};");
 			map.AppendLine ();
 
@@ -4342,14 +4342,8 @@ void SpecializePrepareReturnValue (AutoIndentStringBuilder sb, ObjCMethod method
 			if (isVoid)
 				return;
 
-			switch (rettype) {
-			case "CGRect":
-				body_setup.AppendLine ("{0} res = {{{{0}}}};", rettype);
-				break;
-			default:
-				body_setup.AppendLine ("{0} res = {{0}};", rettype);
-				break;
-			}
+			body_setup.AppendLine ("{0} res = {{ }};", rettype);
+
 			var isArray = returntype is ArrayType;
 			var type = returntype.Resolve () ?? returntype;
 			var retain = method.RetainReturnValue;