[Clang Tablegen] Add llvm_unreachable() to getModifierName()

[msearles]

Summary

Fix internal build failure:

../../../ClangDiagnosticsEmitter.cpp -o ClangDiagnosticsEmitter.o
../../../ClangDiagnosticsEmitter.cpp: In function 'llvm::StringRef 
{anonymous}::getModifierName({anonymous}::ModifierType)':
../../../ClangDiagnosticsEmitter.cpp:495:1: error: control reaches end of non-void function [-Werror=return-type]
}
^

Build failure triggered by git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@332799 91177308-0d34-0410-b5e6-96231b3b80d8

[rsmith]

Please instead add an llvm_unreachable outside the switch, so that we still get warnings on missing switch cases if an enumerator is added.

[msearles]

Ah, yes. Adding llvm_unreachable() is better. Done.

[EricWF]

LGTM.

utils/TableGen/ClangDiagnosticsEmitter.cpp

495

Perhaps this should say "unhandled case" or similar?

[msearles]

Add comment per reviewer suggestion.

utils/TableGen/ClangDiagnosticsEmitter.cpp

case MT_Placeholder:
     return "";
   case MT_ObjCClass:
     return "objcclass";
   case MT_ObjCInstance:
     return "objcinstance";
   case MT_Unknown:
     llvm_unreachable("invalid modifier type");
   }
+  // Unhandled case
+  llvm_unreachable("invalid modifier type");
 }
 
 struct Piece {
   // This type and its derived classes are move-only.
   Piece(PieceKind Kind) : ClassKind(Kind) {}
   Piece(Piece const &O) = delete;
   Piece &operator=(Piece const &) = delete;
   virtual ~Piece() {}