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/CodeGen/GCMetadata.cpp

@@ -148,6 +148,7 @@ static const char *DescKind(GC::PointKind Kind) {
     case GC::PreCall:  return "pre-call";
     case GC::PostCall: return "post-call";
   }
+  llvm_unreachable("Invalid point kind");
 }
 
 bool Printer::runOnFunction(Function &F) {