This CL moves collected source range information out of AST nodes
and into a side table stored on ParseInfo. The side table is only 
created if block coverage is enabled, so there's almost no memory
overhead in the standard case.

Change-Id: I41871b8425ebbc6217d82d3ad26b5fc9e5d68ecb
Reviewed-on: https://chromium-review.googlesource.com/566808
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46590}