[clang] Fix out of line Concept-comparisons of NestedNameSpecifiers (PR #65993)

[Shafik Yaghmour via cfe-commits]

================
@@ -231,14 +231,18 @@ Response HandleFunctionTemplateDecl(const FunctionTemplateDecl *FTD,
                                     MultiLevelTemplateArgumentList &Result) {
   if (!isa<ClassTemplateSpecializationDecl>(FTD->getDeclContext())) {
     NestedNameSpecifier *NNS = FTD->getTemplatedDecl()->getQualifier();
-    const Type *Ty;
-    const TemplateSpecializationType *TSTy;
-    if (NNS && (Ty = NNS->getAsType()) &&
-        (TSTy = Ty->getAs<TemplateSpecializationType>()))
-      Result.addOuterTemplateArguments(const_cast<FunctionTemplateDecl *>(FTD),
-                                       TSTy->template_arguments(),
-                                       /*Final=*/false);
+
+    while (const Type *Ty = NNS ? NNS->getAsType() : nullptr) {
----------------
shafik wrote:

This make sense, I was just looking at the AST: https://godbolt.org/z/P3brjcsMG

and I see that `InnerClass` has:

```console
| |-NestedNameSpecifier TypeSpec 'Base<T>'
```

and I wondering what the prefix is there? Maybe the AST is not telling the whole story?

https://github.com/llvm/llvm-project/pull/65993