PatternRewriter is an implementation detail of the Parser; as such,
there's no need for it to be exposed in parser.h (or even to most
of the Parser). This patch is a cleanup that hides all of PatternRewriter
in pattern-rewriter.cc, exposing only the few helper methods needed
by the rest of Parser in parser.h.

Also removed some duplication between the two PatternRewriter
initialization functions by adding a constructor, and added
a few DCHECKs here and there.

Change-Id: I1dbae8dc0172ff16e40585d0e718d206d2075b3a
Reviewed-on: https://chromium-review.googlesource.com/609365Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47291}