Previously we'd attach source positions to implicit returns that are
generated when leaving an async function with a promise rejection. This
was due to the use of `kNoSourcePosition` on the `end_position` in the
`ReturnStatement` nodes as indicator to pick the return position from
the function literal, instead of really not putting a source position on
that specific `Return` bytecode.

This CL adds a dedicated marker to `ReturnStatement` to express that the
`BytecodeGenerator` should put the return position from the function
literal there instead of overloading the meaning of `kNoSourcePosition`.

Bug: chromium:901819, chromium:782461
Fixed: chromium:1199919, chromium:1201706
Change-Id: I3647e0c3d711e9c3d6ae44606b70ec92ad82e1cf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2859945
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74301}