Add llvm_unreachable at end of fully covered switch

To pacify GCC warning about control reaching end of non-void function.

llvm-svn: 181197
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 2b9d13c..8c65029 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -2514,6 +2514,7 @@
   case ESR_Returned:
     return ESR;
   }
+  llvm_unreachable("Invalid EvalStmtResult!");
 }
 
 // Evaluate a statement.