Simplify CpuIs code to use include from LLVM

LLVM exposes a file in the backend (X86TargetParser.def) that
contains information about the correct list of CpuIs values.

This patch removes 2 of the copied and pasted versions of this
list from clang and instead includes the data from the .def file.

Differential Revision: https://reviews.llvm.org/D40054

llvm-svn: 318234