[lit] Add --xfail-not/LIT_XFAIL_NOT

For example, I need this lately in my CI config:

LIT_XFAIL_NOT='libomptarget :: nvptx64-nvidia-cuda :: unified_shared_memory/api.c'

That test specifies an XFAIL directive, but I get an XPASS result.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D106022