Add more tests for MSVC version handling.

Summary:
This change adds more test cases for the default MSVC compatibility version:
 1. When -fms-extensions is supplied, but -fmsc-version and
    -fms-compatibility-version are not.
 2. With the target triple specifies an MSVC environment, but no other
    -fms* flags.

Reviewers: rnk, llvm-commits

Subscribers: hans, compnerd, amccarth

Differential Revision: https://reviews.llvm.org/D27498

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288997 91177308-0d34-0410-b5e6-96231b3b80d8