[clang] [clang] Use getDefaultArgRange instead of getDefaultArg to retrieve the (PR #79296)

[Haojian Wu via cfe-commits]

hokein wrote:

> Sorry for missing this!
> 
> Would be nice to have a testcase but I see it's hard to get into the crashing state (maybe even not possible via clang). Seems worthwhile to be robust to such conditions and getDefaultArgRange appears less fragile.

This fix was needed for the initial type-alias CTAD implementation, we don't need it anymore in the new implementation :)

We could drop this change, but I think it is worth having it, it is an improvement, and make the code more robust.

https://github.com/llvm/llvm-project/pull/79296