llvm-mc: error when -compress-debug-sections is requested and zlib is not linked

This is a bit of a stab in the dark, since I have zlib on my machine.
Just going to bounce it off the bots & see if it sticks.

Do we have some convention for negative REQUIRES: checks? Or do I just
need to add a feature like I've done here?

llvm-svn: 205050