[AST] Fix a warning

This patch fixes:

  clang/lib/AST/MicrosoftMangle.cpp:1006:11: error: enumeration value
  'S_PPCDoubleDoubleLegacy' not handled in switch [-Werror,-Wswitch]

Author: kazutakahirata

clang/lib/AST/MicrosoftMangle.cpp

@@ -1014,6 +1014,7 @@ void MicrosoftCXXNameMangler::mangleFloat(llvm::APFloat Number) {
   case APFloat::S_x87DoubleExtended: Out << 'X'; break;
   case APFloat::S_IEEEquad: Out << 'Y'; break;
   case APFloat::S_PPCDoubleDouble: Out << 'Z'; break;
+  case APFloat::S_PPCDoubleDoubleLegacy:
   case APFloat::S_Float8E5M2:
   case APFloat::S_Float8E4M3:
   case APFloat::S_Float8E4M3FN: