[X86] Move CPUKind enum from clang to llvm/lib/Support. NFCI

Similar to what some other targets have done. This information
could be reused by other frontends so doesn't make sense to live
in clang.

-Rename CK_Generic to CK_None to better reflect its illegalness.
-Move function for translating from string to enum into llvm.
-Call checkCPUKind directly from the string to enum translation
and update CPU kind to CK_None accordinly. Caller will use CK_None
as sentinel for bad CPU.

I'm planning to move all the CPU to feature mapping out next. As
part of that I want to devise a better way to express CPUs inheriting
features from an earlier CPU. Allowing this to be expressed in a
less rigid way than just falling through a switch. Or using gotos
as we've had to do lately.

Differential Revision: https://reviews.llvm.org/D81439