[NFC][Clang] Fix enumerated mismatch warning (llvm#112816)

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);

```

---------

Co-authored-by: Sirraide <[email protected]>
Co-authored-by: cor3ntin <[email protected]>

Author: 3 people

clang/lib/Parse/ParseDeclCXX.cpp

@@ -3148,11 +3148,13 @@ Parser::DeclGroupPtrTy Parser::ParseCXXClassMemberDeclaration(
   // we did nothing here, but this allows us to issue a more
   // helpful diagnostic.
   if (Tok.is(tok::kw_concept)) {
-    Diag(Tok.getLocation(),
-         DS.isFriendSpecified() || NextToken().is(tok::kw_friend)
-             ? diag::err_friend_concept
-             : diag::
-                   err_concept_decls_may_only_appear_in_global_namespace_scope);
+    Diag(
+        Tok.getLocation(),
+        DS.isFriendSpecified() || NextToken().is(tok::kw_friend)
+            ? llvm::to_underlying(diag::err_friend_concept)
+            : llvm::to_underlying(
+                  diag::
+                      err_concept_decls_may_only_appear_in_global_namespace_scope));
     SkipUntil(tok::semi, tok::r_brace, StopBeforeMatch);
     return nullptr;
   }