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

This fixes the following warnings produced by GCC 9:

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;
      |        ^~~~~~~~~~~~

Differential Revision: https://reviews.llvm.org/D130969