[cpu-detection] Substantial refactor of Host CPU detection code (x86)

Summary:
Following D20970 (committed as r271726).
This is a substantial refactoring of the host CPU detection code.

There is no functionality change intended, but the changes are extensive.

Definitions of architecture types and subtypes are by no means exhaustive or
perfectly defined, but a fair starting point.
Suggestions for futher improvements are welcome.

Reviewers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20988

llvm-svn: 271921