[ Not intended for commit. I'm just posting it to back up an
RFC on cfe-dev. ]
This patch is a hack to add two new "sizeless builtin" types SVInt8_t
and SVInt16_t. The real SVE implementation looks nothing like this;
the patch is just the most direct way of adding two distinct sizeless
types to the type system, so that a follow-on patch can add and test
support for sizeless types in a meaningful way.