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

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

Differential Revision: https://reviews.llvm.org/D47150

llvm-svn: 332854

clang/utils/TableGen/ClangDiagnosticsEmitter.cpp

@@ -492,6 +492,8 @@ static StringRef getModifierName(ModifierType MT) {
   case MT_Unknown:
     llvm_unreachable("invalid modifier type");
   }
+  // Unhandled case
+  llvm_unreachable("invalid modifier type");
 }
 
 struct Piece {