[clang] [clang] Fix code completion crash in lambda trailing requires-clause (PR #206373)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM, this seems reasonable though I expect there are other bugs in the same area, in case anyone wants to be nerd-sniped into follow-up work. For example `getPreferredTypeOfBinaryRHS()` seems to `LHSType` is always valid, similar for `getPreferredTypeOfUnaryArg()`, etc. 

https://github.com/llvm/llvm-project/pull/206373