Reapply [clang] [test] Narrow down MSVC specific behaviours from "any windows"...

Reapply [clang] [test] Narrow down MSVC specific behaviours from "any windows" to only MSVC/clang-cl

This fixes running tests with a toolchain that defaults to a MinGW
target.

After the previous attempt with this patch, this is now changed to
use !defined(__MINGW32__) instead of defined(_MSC_VER) to distinguish
between MSVC and MinGW mode; Clang doesn't define _MSC_VER when invoked
with "clang -cc1" as some of those tests do.

Differential Revision: https://reviews.llvm.org/D149997