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

[Younan Zhang via cfe-commits]

zyn0217 wrote:

> See for example CXXRecordDecl's getLambdaContextDecl function.
> Unfortunately there is an issue where we currently only store this lambda context when we need it for mangling reasons, …

Yeah, I remember we use that to store an ImplicitConceptSpecializationDecl as well, not only for the purpose of mangling but also for lambdas appearing inside a concept. We probably could do the same for the using aliases, though that still needs some tweaking to store template arguments.

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