Avoid warnings from compilers that think you can drop off the end of a fully covered switch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177656 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Driver/ToolChain.cpp

@@ -113,6 +113,8 @@ Tool *ToolChain::getTool(Action::ActionClass AC) const {
   case Action::MigrateJobClass:
     return getClang();
   }
+
+  llvm_unreachable("Invalid tool kind.");
 }
 
 Tool &ToolChain::SelectTool(const JobAction &JA) const {