First ARMTargetParser FIXME, conservatively changing the way we parse CPUs
in the back-end. Still not perfect, with a lot of special cases, but moving
towards a more generic solution.

Moving all logic to the target parser made some unwritten assumptions
about architectures in Clang to break. I've added a lot of architectures
required by Clang, and default to CPUs that Clang believes it should
(and I agree).

I've also added a lot of unit tests, with the correct CPU for each
architecture, and Clang seems to be working correctly, too.

It also became clear that using "unsigned ID" as the argument for the get
methods makes it hard to know what ID, so I also changed the argument names
to match the enum type names.

llvm-svn: 237797