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

[Younan Zhang via cfe-commits]

zyn0217 wrote:

> Either refactor that, or just add an internal 'bool' parameter that controls if the implementation will add the template parameters, and call that from the 'Innermost' path with adding the arguments disabled.

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.

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