This makes for slightly faster rebuilds when touching parser-base.h
(which changes frequently!). Also takes care of an old TODO,
moving CompileTimeValue into its own file under ast/, where it
properly belongs.

BUG=v8:5294

Review-Url: https://codereview.chromium.org/2305883002
Cr-Commit-Position: refs/heads/master@{#39141}