[fixup][Sema] Allow in C to define tags inside enumerations.

[vsapsai]

Summary

Fix for too aggressive error err_type_defined_in_enum introduced in
r313386. Defining tags inside enumerations is prohibited in C++ but
allowed in C.

[rnk]

lgtm

clang/lib/Sema/SemaDecl.cpp

CreateNewDecl:
   //   A type-specifier-seq shall not define a class or enumeration [...].
   if (getLangOpts().CPlusPlus && (IsTypeSpecifier || IsTemplateParamOrArg) &&
       TUK == TUK_Definition) {
     Diag(New->getLocation(), diag::err_type_defined_in_type_specifier)
       << Context.getTagDeclType(New);
     Invalid = true;
   }
 
-  if (!Invalid && TUK == TUK_Definition && DC->getDeclKind() == Decl::Enum) {
+  if (!Invalid && getLangOpts().CPlusPlus && TUK == TUK_Definition &&
+      DC->getDeclKind() == Decl::Enum) {
     Diag(New->getLocation(), diag::err_type_defined_in_enum)
       << Context.getTagDeclType(New);
     Invalid = true;
   }
 
   // Maybe add qualifier info.
   if (SS.isNotEmpty()) {
     if (SS.isSet()) {

clang/test/Sema/enum.c

 typedef enum { NegativeShort = (short)-1 } NegativeShortEnum;
 int NegativeShortTest[NegativeShort == -1 ? 1 : -1];
 
 // PR24610
 enum Color { Red, Green, Blue }; // expected-note{{previous use is here}}
 typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type that does not match previous declaration}}
 
 // PR28903
+// In C it is valid to define tags inside enums.
 struct PR28903 {
   enum {
-    PR28903_A = (enum { // expected-error-re {{'enum PR28903::(anonymous at {{.*}})' cannot be defined in an enumeration}}
+    PR28903_A = (enum {
       PR28903_B,
       PR28903_C = PR28903_B
     })0
   };
   int makeStructNonEmpty;
 };