Politely inform GCC that we will never fall off the end of that switch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127783 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGExprConstant.cpp b/lib/CodeGen/CGExprConstant.cpp
index 445bca0..a3c4536 100644
--- a/lib/CodeGen/CGExprConstant.cpp
+++ b/lib/CodeGen/CGExprConstant.cpp
@@ -618,6 +618,7 @@
     case CK_FloatingCast:
       return llvm::ConstantExpr::getFPCast(C, destType);
     }
+    llvm_unreachable("Invalid CastKind");
   }
 
   llvm::Constant *VisitCXXDefaultArgExpr(CXXDefaultArgExpr *DAE) {