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

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

> But perhaps at least you could add named accessors to DIDerivedType for the template params, same as DIGlobalVariable/DICompositeType have?

Done. I couldn't add an assert (`getTag() == dwarf::DW_TAG_template_alias`) without including `Dwarf.h`. I've chosen to not do that as it looks like its been avoided (but possibly just not really needed here before), but I'd be happy to add it if the extra compile time isn't worth worrying about.

> (oh, and in case anyone hasn't mentioned it already - this would generally be committed in smaller pieces upstream - adding the LLVM functionality first, then adding clang patches that use that functionality)

Sure, no problem, I'll open separate pull requests once this has been accepted then.

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I think I've addressed all the concerns raised now except for the variadic issue I mentioned, which I'll look at tomorrow.

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