[X86] Clzero flag addition and inclusion under znver1

1. Adds the command line flag for clzero.
2. Includes the clzero flag under znver1.
3. Defines the macro for clzero.
4. Adds a new file which has the intrinsic definition for clzero instruction.

Patch by Ganesh Gopalasubramanian with some additional tests from me.

Differential revision: https://reviews.llvm.org/D29386

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294559 91177308-0d34-0410-b5e6-96231b3b80d8