CodeGen: tweak struct ABI handling

Cygwin and MinGW fail to conform to the underlying system's structure passing
ABI.  Make the check more precise to ensure that we correctly generate code for
the itanium environment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222626 91177308-0d34-0410-b5e6-96231b3b80d8