[clang]Silence warning in MicrosoftCXXABI.cpp

[clang]Silence warning in MicrosoftCXXABI.cpp

Silence warning with gcc 9.3 about:

[1/351] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/MicrosoftCXXABI.cpp.o
../../clang/lib/AST/MicrosoftCXXABI.cpp:57:12: warning: 'virtual unsigned int {anonymous}::MicrosoftNumberingContext::getManglingNumber(const clang::VarDecl*, unsigned int)' was hidden [-Woverloaded-virtual]
   57 |   unsigned getManglingNumber(const VarDecl *VD,
      |            ^~~~~~~~~~~~~~~~~
../../clang/lib/AST/MicrosoftCXXABI.cpp:80:12: warning:   by 'virtual unsigned int {anonymous}::MSHIPNumberingContext::getManglingNumber(const clang::TagDecl*, unsigned int)' [-Woverloaded-virtual]
   80 |   unsigned getManglingNumber(const TagDecl *TD,
      |            ^~~~~~~~~~~~~~~~~

Change-Id: Ia519e77c6454eb020228478dd6498eaf7864dae8