clang-cl: Warn about overriding /MD with /MT etc.

This also bakes the /M options into a separate option group to make
them easier to refer to from the code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190529 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index 9bbbaae..7461e99 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -1011,8 +1011,8 @@
     Arg *Previous = *it++;
     bool ShowNote = false;
     while (it != ie) {
-      Diag(clang::diag::warn_drv_overriding_t_option) << Previous->getSpelling()
-          << (*it)->getSpelling();
+      Diag(clang::diag::warn_drv_overriding_flag_option)
+          << Previous->getSpelling() << (*it)->getSpelling();
       Previous = *it++;
       ShowNote = true;
     }