To improve the Torque debugging experience, we can add source positions
for each line. This information is carried through the generated
CSA code (in <output directory>/gen/torque-generated/*.cc) and
embedded as SourcePositions in the Code object.

At snapshot time, these SourcePositions are stripped from the Code
object and turned into platform-appropriate line number debug
information.

At this time on Linux, you'll need to build with "is_clang=false"
in order to use GCC, because crucial steps are missing in Clang's
ability to convey the information into the binary successfully.

This CL also introduces a flag to control the existing source
information in CSA code. --enable-source-at-csa-bind is now set
to false by default because it's a bit confusing to "hop" between
source lines in .TQ files and in .CC files. I expect to continue
making adjustments there, as I want to provide helpful
debugging aids at the CSA level as well as the Torque level.
The current configuration prioritizes Torque.

A detailed guide on usage to follow (also on v8.dev).

Bug: v8:8418
Change-Id: Ib4226877ce4cae451bb4d0c546927e89f4e66b58
Reviewed-on: https://chromium-review.googlesource.com/c/1475473Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59636}