Print an error message when someone tries -integrated-as on an unsupported target.

Print an error message when someone tries -integrated-as on an unsupported target.

- The COFF backend doesn't support MingW/Cygwin at the moment, it'll report an
  error, but it's still much better than random assertions from the MachO backend.
- We want to make ELF the default eventually, it's what the majority of targets use.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110197 91177308-0d34-0410-b5e6-96231b3b80d8