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

[Richard Smith via cfe-commits]

https://github.com/zygoloid approved this pull request.

LGTM too. I do wonder if we can make tablegen generate the data directly in the desired format here (perhaps with some additional `.td` files to define exactly which files we want to include in which targets and to define the placeholder for the target-independent builtins), but I don't think this (huge) improvement needs to block on that.

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