[SKX] Enabling SKX target (Skylake server chip) 
a) add SKX support to Clang driver;
b) add tests for SKX target and AVX512BW, AVX512DQ, AVX512VL features into clang driver tests

Patch by Zinovy Nis <zinovy.y.nis@intel.com>


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214306 91177308-0d34-0410-b5e6-96231b3b80d8