[clang] [clang-tools-extra] [lldb] [clang] Reland: Instantiate alias templates with sugar (PR #101858)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

One possibility here is that if you have the type, then you will have a template specialization type for the template alias, and that gives you the template arguments used to specialize the alias. Which should answer this need.

But of course, if you have a lossy semantic adjustment which makes you lose top level sugar, then you might not have the template specialization type anymore, but this will also apply to Subst* nodes, which can also be lost in these cases.

But you might have noticed already that type sugar cannot be reliably depended upon.

@zygoloid thoughts?

https://github.com/llvm/llvm-project/pull/101858