[clang] [libclang] Add accessors for template parameters and fix parameter pack handling (PR #183504)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I've got no problem with this, but most of this is not in clang itself, so I don't know enough to review the lib-clang stuff.  Everything I understand looks right, but others will have to approve.

https://github.com/llvm/llvm-project/pull/183504