Fix erroneous warning on enum struct parameters.

Bug: issue #991
Test: new test case

compiler/semantics.cpp

@@ -730,7 +730,9 @@ bool Semantics::CheckBinaryExpr(BinaryExpr* expr) {
 
             // If it's an outparam, also mark it as read.
             if (sym->vclass() == sARGUMENT &&
-                (sym->type()->isReference() || sym->type()->isArray()))
+                (sym->type()->isReference() ||
+                 sym->type()->isArray() ||
+                 sym->type()->isEnumStruct()))
             {
                 markusage(sym, uREAD);
             }

tests/compile-only/ok-enum-struct-param-implicit-read.sp

@@ -0,0 +1,18 @@
+// warnings_are_errors: true
+
+enum struct Foo
+{
+    int value;
+}
+
+void FunctionWithStructParam(Foo param)    // warning 204: symbol is assigned a value that is never used: "param"
+{
+    Foo bar;
+    bar.value = 1;
+    param = bar;
+}
+
+public main() {
+    Foo foo;
+    FunctionWithStructParam(foo);
+}