C API header xcvsimd and type fixes

This patch adds the C API header file for the xcvsimd Clang builtins. It adds the CodeGen test that checks that the wrapper functions defined in the header turn into the right LLVM instrinsic when called.

The patch also adds a fix for the types of the operands of some simd builtins that were wrongly declared as signed instead of unsigned or as not constant arguments. The CodeGen tests are updated accordingly.

Finally the patch adds the cmake support for the header file. This needs to follow the analogous patch for the xcvbitmanip builtins as that one includes the basic cmake support to add all the headers.