[Driver][CodeGen] Properly handle -fsplit-machine-functions for fatbinary compilation.

When building a fatbinary, the driver invokes the compiler multiple
times with different "--target". (For example, with "-x cuda
--cuda-gpu-arch=sm_70" flags, clang will be invoded twice, once with
--target=x86_64_...., once with --target=sm_70) If we use
-fsplit-machine-functions or -fno-split-machine-functions for such
invocation, the driver reports an error.

This CL changes the behavior so:

  - "-fsplit-machine-functions" is now passed to all targets, for non-X86
    targets, the flag is a NOOP and causes a warning.

  - "-fno-split-machine-functions" now negates -fsplit-machine-functions (if
    -fno-split-machine-functions appears after any -fsplit-machine-functions)
    for any target triple, previously, it causes an error.

  - "-fsplit-machine-functions -Xarch_device -fno-split-machine-functions"
    enables MFS on host but disables MFS for GPUS without warnings/errors.

  - "-Xarch_host -fsplit-machine-functions" enables MFS on host but disables
    MFS for GPUS without warnings/errors.

Reviewed by: xur, dhoekwater

Differential Revision: https://reviews.llvm.org/D157750