This reduces the memory consumption of SourcePositionTable by ca. 2/3.
Over Octane, this reduces the source position table memory consumption
from ~370kB to ~115kB, which makes it ca. 10% of the total bytecode size
(~1.1MB)

BUG=

Review URL: https://codereview.chromium.org/1704943002

Cr-Commit-Position: refs/heads/master@{#34250}