[clang] [Clang] No longer require complete types with __builtin_launder (PR #91070)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane I think we should rebase and merge, what do you think?

Feel free, I think that we need to do this.  I'd very much like an improved commit message.  I'm no longer convinced we have to force-instantiate, but I'm not sure I fully Grok that discussion anymore.

https://github.com/llvm/llvm-project/pull/91070