[TableGen] Use a table to lookup MVE intrinsic names

Summary:
Speeds up compilation of SemaDeclAttr.cpp by nine seconds:
  0m49.555s - > 0m40.249s

Reviewers: simon_tatham, dmgreen, ostannard, MarkMurrayARM

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72984