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

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Sirraide wrote:

> in _debug mode_ only

Or rather than in debug mode perhaps if assertions are enabled. Not sure what the expected slowdown for an assertions build is usually.

@AaronBallman for opinions

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