[clang] [test] Narrow down an MSVC specific behaviour to only not covever MinGW

This uses the same logic as in c2b256a9;
we can't check defined(_MSC_VER) invoked as %clang_cc1, therefore
check for !defined(__MINGW32__) instead.

This fixes the same issue in a new testcase that was added after this
issue was fixed last time in c2b256a9.

Differential Revision: https://reviews.llvm.org/D151661