[COFF, Mach-O] Include -mllvm options in thinlto cache key

Like D134013, but for COFF and Mach-O.

Also expand the ELF test a bit. I at first didn't realize that `getValue()` for
`-mllvm -foo=bar` would return `-foo=bar` instead of just `bar`, and so
I wrote the test to check if we indeed get this wrong. We don't, but
having the test for it seems nice, so I'm including it.

Differential Revision: https://reviews.llvm.org/D137971