Make parseBitcodeFile return an ErrorOr<Module *>.
lit.cfg: Don't expand %ms_abi_triple to non-X86 triples

This is a follow-up to r199260. On ARM hosts, we were attempting to run
tests with triples such as armv7l-unknown-win32. This expands that fix to
cover all non-x86 targets since we only support MS ABI on x86.