[clang] [Clang][Sema] Allow access to a public template alias declaration that refers to friend's private nested type (PR #83847)

[Younan Zhang via cfe-commits]

================
@@ -4342,10 +4342,17 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     InstantiatingTemplate Inst(*this, TemplateLoc, Template);
     if (Inst.isInvalid())
       return QualType();
+    if (!AliasTemplate->getDeclContext()->isFileContext()) {
+      ContextRAII SavedContext(*this, AliasTemplate->getDeclContext());
----------------
zyn0217 wrote:

nit: This can be simplified using `std::optional`:
```cpp
std::optional<ContextRAII> C;
if (!A.getDeclContext()->isFileContext())
  C.emplace(*this, A->getDeclContext());
```

https://github.com/llvm/llvm-project/pull/83847