This CL changes 'Value' to use an 'Identifier' for its name, where
the source position represents the point where it is defined. This is
used to support "goto definition" for constants and extern constants.

R=tebbi@chromium.org

Bug: v8:8880
Change-Id: Ifb9ff08b36cbd9fb2691dbae579d2df29edd651d
Reviewed-on: https://chromium-review.googlesource.com/c/1495986Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59977}