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

[Erich Keane via cfe-commits]

erichkeane wrote:

> Should we try to force an instantiation in codegen for complete-but-not-yet-instantiated classes?

I think we have to here.  I believe we can do this by changing the `RequireCompleteType` to pass a null `diagnoser`.  

See this overload: https://clang.llvm.org/doxygen/classclang_1_1Sema.html#ac457a5af05f2ffc2cf6286f398d9201d (RequireCompleteType() [1/5])

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