For a script '()=>42', the anonymous arrow function has both start and
end position the same as the script function itself. This causes issues
when sorting the SourcePositionEvents of the function, in two ways:

  * If the start positions are the same, we should order by *furthest*
    end position to ensure the stack is in the right order
  * If both start and end are the same, we need to order by function
    literal id to make sure that start order and end order are inversed.

Also, MapLiterals assumes that start+end position uniquely identifies a
function, which is false in this case, so we process the top-level
script function separately in MapLiterals.

Change-Id: I2b2185dc2825018b7ea44c7d0918238e9b1dd972
Reviewed-on: https://chromium-review.googlesource.com/1141741
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54525}