[clang] [Clang][NFC] Remove TemplateArgumentList::OnStack (PR #79760)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane I ended up writing a python script that compiles the TUs in `clang/lib` with `-print-stats` and then sums up the total bytes allocated by `ASTContext`. Ended up with a 0.4% decrease in memory usage.
> 
> This does however rely on `compile_commands.json`, and boost does not have a great way to generate this for every subproject... Do you think the results from compiling clang are sufficient?

Yeah, I think that is sufficient, find a way to write that as prose in the release note, and i'm happy with it.

https://github.com/llvm/llvm-project/pull/79760