[clang] [Sema] Fast-path simple plain auto deduction in DeduceAutoType (PR #188196)

[via cfe-commits]

Sirraide wrote:

> Also I think @Sirraide is requesting for debug-mode-only assertions but not assertions that may also apply to release mode build

It might be a good idea to put this behind `LLVM_ENABLE_EXPENSIVE_CHECKS`

https://github.com/llvm/llvm-project/pull/188196