[clang] [HLSL] Fix GCC warnings about virtual methods that are hidden

[mstorsjo]

Summary

This fixes the following warnings produced by GCC 9:

[...] Building CXX object tools/clan...gSema.dir/HLSLExternalSemaSource.cpp.o
In file included from ../tools/clang/include/clang/Sema/ExternalSemaSource.h:15,

from ../tools/clang/include/clang/Sema/HLSLExternalSemaSource.h:17,
from ../tools/clang/lib/Sema/HLSLExternalSemaSource.cpp:12:

../tools/clang/include/clang/AST/ExternalASTSource.h:211:16: warning: ‘virtual void clang::ExternalASTSource::CompleteType(clang::ObjCInterfaceDecl*)’ was hidden [-Woverloaded-virtual]

211 |   virtual void CompleteType(ObjCInterfaceDecl *Class);
    |                ^~~~~~~~~~~~

In file included from ../tools/clang/lib/Sema/HLSLExternalSemaSource.cpp:12:
../tools/clang/include/clang/Sema/HLSLExternalSemaSource.h:49:8: warning: by virtual void clang::HLSLExternalSemaSource::CompleteType(clang::TagDecl*)’ [-Woverloaded-virtual]

49 |   void CompleteType(TagDecl *Tag) override;
   |        ^~~~~~~~~~~~

[beanz]

LGTM. This is a really unfortunate interface that causes this :(.

clang/include/clang/Sema/HLSLExternalSemaSource.h

public:
   /// Initialize the semantic source with the Sema instance
   /// being used to perform semantic analysis on the abstract syntax
   /// tree.
   void InitializeSema(Sema &S) override;
 
   /// Inform the semantic consumer that Sema is no longer available.
   void ForgetSema() override { SemaPtr = nullptr; }
 
+  using ExternalASTSource::CompleteType;
   /// Complete an incomplete HLSL builtin type
   void CompleteType(TagDecl *Tag) override;
 };
 
 } // namespace clang
 
 #endif // CLANG_SEMA_HLSLEXTERNALSEMASOURCE_H