[Sema]Use tag name lookup for class names (#112166)

This PR would fix #16855 .

The correct lookup to use for class names is Tag name lookup,
because it does not take namespaces into account. The lookup before
does and because of this some valid programs are not accepted.

An example scenario of a valid program being declined is when you have a struct (let's call it `y`) inheriting from another struct with a name `x` but the struct `y` is in a namespace that is also called `x`:
```
struct x
{};

namespace
{
    namespace x
    {
        struct y : x
        {};
    }
}
```

This shall be accepted because: 
```
C++ [class.derived]p2 (wrt lookup in a base-specifier): The lookup for
  // the component name of the type-name or simple-template-id is type-only.
```

Author: spaits

clang/docs/ReleaseNotes.rst

@@ -516,6 +516,7 @@ Bug Fixes to C++ Support
 - Fixed a bug in lambda captures where ``constexpr`` class-type objects were not properly considered ODR-used in
   certain situations. (#GH47400), (#GH90896)
 - Fix erroneous templated array size calculation leading to crashes in generated code. (#GH41441)
+- During the lookup for a base class name, non-type names are ignored. (#GH16855)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -357,10 +357,13 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
       return nullptr;
   }
 
-  // FIXME: LookupNestedNameSpecifierName isn't the right kind of
-  // lookup for class-names.
-  LookupNameKind Kind = isClassName ? LookupNestedNameSpecifierName :
-                                      LookupOrdinaryName;
+  // In the case where we know that the identifier is a class name, we know that
+  // it is a type declaration (struct, class, union or enum) so we can use tag
+  // name lookup.
+  //
+  // C++ [class.derived]p2 (wrt lookup in a base-specifier): The lookup for
+  // the component name of the type-name or simple-template-id is type-only.
+  LookupNameKind Kind = isClassName ? LookupTagName : LookupOrdinaryName;
   LookupResult Result(*this, &II, NameLoc, Kind);
   if (LookupCtx) {
     // Perform "qualified" name lookup into the declaration context we

clang/test/CXX/class.derived/p2.cpp

@@ -6,4 +6,16 @@ namespace PR5840 {
   struct Base {};
   int Base = 10;
   struct Derived : Base {};
-}
+} // namespace PR5840
+
+namespace issue_16855 {
+  struct x {};
+  namespace
+  {
+      namespace x
+      {
+          struct y : x
+          {};
+      } // namespace x
+  }
+} // namespace issue_16855