[NFC][Clang] Fix enumerated mismatch warning

This is one of the many PRs to fix errors with LLVM_ENABLE_WERROR=on. Built by GCC 11.

Fix warning:
llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:3153:14: error: enumerated mismatch in conditional expression: ‘clang::diag::<unnamed enum>’ vs ‘clang::diag::<unnamed enum>’ [-Werror=enum-compare]
 3152 |          DS.isFriendSpecified() || NextToken().is(tok::kw_friend)
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 3153 |              ? diag::err_friend_concept
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~
 3154 |              : diag::
      |              ~~~~~~~~
 3155 |                    err_concept_decls_may_only_appear_in_global_namespace_scope);

Author: jsji

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -983,9 +983,6 @@ def warn_cxx23_variadic_friends : Warning<
   "variadic 'friend' declarations are incompatible with C++ standards before C++2c">,
   DefaultIgnore, InGroup<CXXPre26Compat>;
 
-def err_friend_concept : Error<
-  "friend declaration cannot be a concept">;
-
 // C++11 default member initialization
 def ext_nonstatic_member_init : ExtWarn<
   "default member initializer for non-static data member is a C++11 "

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3020,6 +3020,8 @@ def err_c23_constexpr_pointer_not_null : Error<
   "constexpr pointer initializer is not null">;
 
 // C++ Concepts
+def err_friend_concept : Error<
+  "friend declaration cannot be a concept">;
 def err_concept_decls_may_only_appear_in_global_namespace_scope : Error<
   "concept declarations may only appear in global or namespace scope">;
 def err_concept_no_parameters : Error<