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

[via cfe-commits]

cor3ntin wrote:

actually, I talked to @jwakely and the standard has blanket wording to make that...UB.

so we could merge it as a compiler extension as it seems useful... however my initial read of the wording was wrong...

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