Add support for System z vector language extensions

The z13 vector facility has an associated language extension,
closely modeled on AltiVec/VSX.  The main differences are:

- vector long, vector float and vector pixel are not supported

- vector long long and vector double are supported (like VSX)

- comparison operators return a vector rather than a scalar integer

- shift operators behave like the OpenCL shift operators

- vector bool is only supported as argument to certain operators;
  some operators allow mixing a bool with a non-bool vector 

This patch adds clang support for the extension.  It is closely modelled
on the AltiVec support.  Similarly to the -faltivec option, there's a
new -fzvector option to enable the extensions (as well as an -mzvector
alias for compatibility with GCC).  There's also a separate LangOpt.

The extension as implemented here is intended to be compatible with
the -mzvector extension recently implemented by GCC.

Based on a patch by Richard Sandiford.

Differential Revision: http://reviews.llvm.org/D11001

llvm-svn: 243642