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

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.

Reviewers: efriedma, rovka, SjoerdMeijer, rsandifo-arm, rengolin

Reviewed By: SjoerdMeijer

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75470