Reland D75470 [SVE] Auto-generate builtins and header for svld1.

Reworked the patch to avoid sharing a header (SVETypeFlags.h) between
include/clang/Basic and utils/TableGen/SveEmitter.cpp. Now the patch
generates the enum/flags which is included in TargetBuiltins.h.

Also renamed one of the SveEmitter options to be in line with MVE.

Summary:

This is a first patch in a series for the SveEmitter to generate the arm_sve.h
header file and builtins.

I've tried my best to strip down this patch as best as I could, but there
are still a few changes that are not necessarily exercised by the load intrinsics
in this patch, mostly around the SVEType class which has some common logic to
represent types from a type and prototype string. I thought it didn't make
much sense to remove that from this patch and split it up.