[liveedit] Use start position in function lookup

Instead of looking up functions by their function literal id (which can
be slow now that function id involves a linear search for compiled
functions), we key the lookup by the function's start position.

This means that the script+literal id swapping to find equivalent
unchanged functions during constant pool patching no longer works -- we
could replace it by fixing up the start position of the redundant new
function, but instead we just build up a side-table mapping (new) start
positions to function literal ids, and use that function literal id to
find the old function in the script's SFI list.

Change-Id: I10bfce6c39665cba063e0ddbc8fd38a6f5fd5513
Reviewed-on: https://chromium-review.googlesource.com/1140169Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54542}