Add 'llvm_unreachable' to passify GCC's understanding of the constraints

of several newly un-defaulted switches. This also helps optimizers
(including LLVM's) recognize that every case is covered, and we should
assume as much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147861 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCExpr.cpp

@@ -17,6 +17,7 @@
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
@@ -216,6 +217,7 @@ StringRef MCSymbolRefExpr::getVariantKindName(VariantKind Kind) {
   case VK_Mips_GOT_PAGE: return "GOT_PAGE";
   case VK_Mips_GOT_OFST: return "GOT_OFST";
   }
+  llvm_unreachable("Invalid variant kind");
 }
 
 MCSymbolRefExpr::VariantKind