[Power9] Builtins for ELF v.2 ABI conformance - front end portion

This patch corresponds to review:
https://reviews.llvm.org/D24397

It adds the __POWER9_VECTOR__ macro and the -mpower9-vector option along with
a number of altivec.h functions (refer to the code review for a list).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282481 91177308-0d34-0410-b5e6-96231b3b80d8