[clang] [llvm] [Clang] Emit DW_TAG_template_alias for template aliases (PR #87623)

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

I've rebased this so now this pull request contains only the Clang changes (LLVM side is here #88943).

There's a discussion starting [here](https://github.com/llvm/llvm-project/pull/87623#discussion_r1567968127) that discusses whether or not we should include defaulted arguments in the list. The very short summary is that I tried this by scraping the defaults from the template parameter list, but it turns out this is difficult in the face of parameter defaults that are dependent types and values. So the current implementation ignores defaulted arguments, i.e., doesn't include them in the argument list (and as a consequence also omits empty parameter packs that come after defaulted arguments).

This is not ideal, but not a regression from the DW_TAG_typedef names which also do not include defaulted arg values.

https://github.com/llvm/llvm-project/pull/87623