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

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane How does this sound?
> 
> > Removed support for constructing on-stack `TemplateArgumentList`s; interfaces should instead use `ArrayRef<TemplateArgument>` to pass template arguments. Transitioning internal uses to `ArrayRef<TemplateArgument>` reduces AST memory usage by 0.4% when compiling clang, and is expected to show similar improvements on other workloads.

See edited quote^

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