Add the map name clashing syntax test

Protoc disallows using the <field name>Entry if the respective
<field name> map field is defined. But meanwhile it allows using
<field name>_Entry as the field name, which ends up as
<field name>Entry in the generated code. The difference is in the
letter case is used for the alias we add for map entries and the
actual field name.

As the conclusion - test passes, but we should keep it in our test
suite to ensure we compliant with the protoc rules.

Pick-to: 6.8 6.9
Change-Id: I8a8d6796b6c5564b211bd11fdd23102b0031031e
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>

tests/auto/shared/data/proto/syntax.proto

@@ -138,3 +138,9 @@ message NameClashingRepeated {
 
 message NameClashing_Repeated {
 }
+
+message NameClashingMap {
+    map<int32, int32> field1 = 1;
+    int32 Field1_Entry = 2;
+    int32 Field2Entry = 3;
+}