Disable warning about MSVC not found.

When this test runs on bots that are configured to default
to MSVC, but MSVC isn't actually installed, we can emit a
warning that MSVC is not found.  Since MSVC isn't actually
needed for this test to succeed, just disable this warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297858 91177308-0d34-0410-b5e6-96231b3b80d8