[clang] [Clang] Add BuiltinTemplates.td to generate code for builtin templates (PR #123736)

[Erich Keane via cfe-commits]

erichkeane wrote:

> Gentle ping @erichkeane

Thanks for the ping, I'm just getting back today from WG21, so I am still catching up on reviews.  

https://github.com/llvm/llvm-project/pull/123736