The issue is that the shorthand if let syntax injects an implicit
expression: https://github.com/apple/swift/pull/40694/ in ParseStmt and
that the 'diagnoseUnhandledAsyncSite' explicitly avoids reporting errors
in implicit expressions.
This change is that we don't mark the implicit declref code emitted by
the `if let prop` as implicit anymore, and this way the reporting works
out as expected.
Added some tests covering this as well as properly erroring out for the
nonexistent syntax of shortand + awaiting which doesn't exist, and we
properly error on it.
Resolves rdar://126169564
