[Clang] Fix an incorrect assumption on getTemplatedDecl() (#131559)

Since a68d20e, we've been calling HandleDelayedAccessCheck() for
concept declarations when the declaration contains invalid member
accesses.

However, a concept declaration is TemplateDecl such that doesn't contain
any TemplatedDecl.

Fixes #131530

Author: zyn0217

clang/lib/Sema/SemaAccess.cpp

@@ -1518,8 +1518,8 @@ void Sema::HandleDelayedAccessCheck(DelayedDiagnostic &DD, Decl *D) {
   } else if (FunctionDecl *FN = dyn_cast<FunctionDecl>(D)) {
     DC = FN;
   } else if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D)) {
-    if (isa<DeclContext>(TD->getTemplatedDecl()))
-      DC = cast<DeclContext>(TD->getTemplatedDecl());
+    if (auto *D = dyn_cast_if_present<DeclContext>(TD->getTemplatedDecl()))
+      DC = D;
   } else if (auto *RD = dyn_cast<RequiresExprBodyDecl>(D)) {
     DC = RD;
   }

clang/test/SemaCXX/concept-crash-on-diagnostic.cpp

@@ -36,3 +36,15 @@ void function() {
 // expected-note@#4 {{candidate template ignored: constraints not satisfied [with IteratorL = Object *, IteratorR = Object *]}}
 // We don't know exactly the substituted type for `lhs == rhs`, thus a placeholder 'expr-type' is emitted.
 // expected-note@#3 {{because 'convertible_to<expr-type, bool>' would be invalid}}
+
+namespace GH131530 {
+
+class foo {
+  struct bar {}; // expected-note {{implicitly declared private}}
+};
+
+template <typename T>
+concept is_foo_concept = __is_same(foo::bar, T);
+// expected-error@-1 {{'bar' is a private member of 'GH131530::foo'}}
+
+}