Re-work the Clang system for classifying Intel x86 CPUs to use their
basic microarchitecture names, and add support (with tests) for parsing
all of the masic microarchitecture names for CPUs documented to be
accepted by GCC with -march. I didn't go back through the 32-bit-only
old microarchitectures, but this at least brings the recent architecture
names up to speed. This is essentially the follow-up to the LLVM commit
r223769 which did similar cleanups for the LLVM CPUs.

One particular benefit is that you can now use -march=westmere in Clang
and get the LLVM westmere processor which is a different ISA variant (!)
and so quite significant.

Much like with r223769, I would appreciate the Intel folks carefully
thinking about the macros defined, names used, etc for the atom chips
and newest primary x86 chips. The current patterns seem quite strange to
me, especially here in Clang.

Note that I haven't replicated the per-microarchitecture macro defines
provided by GCC. I'm really opposed to source code using these rather
than using ISA feature macros.