[clang] [Clang][NFCI] Slightly refactor getTemplateInstantiationArgs() (PR #102922)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> Thinking twice, I think this is probably feasible to go. As an internal flag wouldn't be exposed to users, but we might have to refactor the function a lot more.

Yep. Just don't forget to assert in case the template arguments wouldn't get added, to diagnose users passing Innermost with a declaration which doesn't take template arguments :)

https://github.com/llvm/llvm-project/pull/102922