Fix up compression related test cases

Fallout from r205261, ensure it doesn't matter how we disable compressed
debug info, even if zlib is missing and that we warn when we don't have
zlib and don't warn when we do, all while silently suppressing these
tests on the systems they weren't intended for...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205271 91177308-0d34-0410-b5e6-96231b3b80d8