[clang] Switch builtin strings to use string tables (PR #118734)

[Chandler Carruth via cfe-commits]

chandlerc wrote:

> Confirmed that this works on GCC now. I'd suggest to replace the use of StringLiteral::size with plain sizeof(). The build time overhead of going through StringLiteral here is substantial.

Sure. I was initially worried about the subtlety of this use of `sizeof`, but that was before everything got nicely factored into macros. And yeah, the compile time cost is unfortunate.

Will merge once updated (including this fix) and builds passing.

https://github.com/llvm/llvm-project/pull/118734