[Clang] Don't form a type constraint if the concept is invalid

After 0dedd6f and 03229e7, invalid concept declarations might
lack expressions for evaluation and normalization. This could make
it crash in certain scenarios, apart from the one of evaluation
concepts showed in 03229e7, there's also an issue when checking
specializations where the normalization also relies on a non-null expression.

This patch prevents that by avoiding building up a type constraint
in such situations, thereafter the template parameter wouldn't
have a concept specialization of a null expression.

With this patch, the assumption in ASTWriterDecl is no longer valid.
Namely, HasConstraint and TypeConstraintInitialized must now represent
different meanings for both source fidelity and semantic requirements.

Author: zyn0217

clang/lib/Sema/SemaTemplate.cpp

@@ -4557,6 +4557,9 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
                              const TemplateArgumentListInfo *TemplateArgs) {
   assert(NamedConcept && "A concept template id without a template?");
 
+  if (NamedConcept->isInvalidDecl())
+    return ExprError();
+
   llvm::SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
   if (CheckTemplateArgumentList(
           NamedConcept, ConceptNameInfo.getLoc(),

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1951,7 +1951,6 @@ void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   Record.push_back(D->wasDeclaredWithTypename());
 
   const TypeConstraint *TC = D->getTypeConstraint();
-  assert((bool)TC == D->hasTypeConstraint());
   if (TC) {
     auto *CR = TC->getConceptReference();
     Record.push_back(CR != nullptr);

clang/test/SemaTemplate/concepts.cpp

@@ -1165,3 +1165,15 @@ concept C = invalid; // expected-error {{use of undeclared identifier 'invalid'}
 bool val2 = C<int>;
 
 } // namespace GH109780
+
+namespace GH121980 {
+
+template <class>
+concept has_member_difference_type; // expected-error {{expected '='}}
+
+template <has_member_difference_type> struct incrementable_traits; // expected-note {{declared here}}
+
+template <has_member_difference_type Tp>
+struct incrementable_traits<Tp>; // expected-error {{not more specialized than the primary}}
+
+}