[SystemZ] Add support for vecintrin.h vector built-in functions

This patch adds support for the System Z vector built-in functions.
The API-defined header file has the name vecintrin.h.

The user-level functions are defined in the same style as the clang
version of altivec.h, making heavy use of the __overloadable__ and
__always_inline__ attributes.  Where possible the functions expand to
generic operations rather than specific built-in functions, in the hope
that that form can be optimised better.

Where a built-in routine is specified to require an immediate integer
argument, the __enable_if__ attribute is used to verify the argument is
in fact constant and in the appropriate range.

Based on a patch by Richard Sandiford.

llvm-svn: 243643