Request for review: Preserve async-let typed errors (#91964)

Hi everyone! I'm a new contributor to the Swift project and I'm not entirely familiar with the usual PR review process or how long it typically takes to get feedback. I have grown increasingly frustrated by the fact that async let doesn't preserve the thrown error type of a typed-throwing function. After looking into it, I discovered that this had already been reported some time ago in issue #76169 but I couldn't find any indication that anyone had started working on a fix. So I decided to giv...