Fix LLVM's handling and detection of skylake and cannonlake CPUs

Summary:
 - Rename `"skylake"` == SkylakeServerProc to `"skylake-avx512"`
 - Change `"skylake"` to denote SkylakeClientProc
 - Fix the detection of cpu family 6 and model 94 to be
   SkylakeClientProc instead of SkylakeServerProc
 - Remove the `"cnl"` for CannonLake

Reviewers: craig.topper, delena

Subscribers: zansari, echristo, qcolombet, RKSimon, spatel, DavidKreitzer, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D17090

llvm-svn: 261482