Get default -fms-compatibility-version from cl.exe's version

Get default -fms-compatibility-version from cl.exe's version

-fms-compatibility-version was defaulting to 18 (VS 2013), which is a pain if your environment is pointing to version 19 (VS 2015) libraries.

If cl.exe can be found, this patch uses its version number as the default instead. It re-uses the existing code to find the Visual Studio binaries folder and WinAPI methods to check its version. You can still explicitly specify a compatibility version on the command line. If you don't have cl.exe, this should be a no-op and you'll get the old default of 18.

This affected the tests, which assumed that if you didn't specific a version, that it would default to 18, but this won't be true for all machines. So a couple test cases had to be eliminated and a couple others had to be tweaked to allow for various outputs.

Addresses: https://llvm.org/bugs/show_bug.cgi?id=27215

Differential Revision: http://reviews.llvm.org/D20136

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269515 91177308-0d34-0410-b5e6-96231b3b80d8