AMDGPU: Partially move target handling code from clang to TargetParser

A future change in clang necessitates access of this information
from the driver, so move this into a common place.

Try to mimic something resembling the API the other targets are
using here.

One thing I'm uncertain about is how to split amdgcn and r600
handling. Here I've mostly duplicated the functions for each,
while keeping the same enums. I think this is a bit awkward
for the features which don't matter for amdgcn.

It's also a bit messy that this isn't a complete set of
subtarget features. This is just the minimum set needed
for the driver code. For example building the list of
subtarget feature names is still in clang.

llvm-svn: 340291