This uses the PIMPL idiom to hide the implementation of
{CompilationState} while still allowing to call methods on
{CompilationState} using the externally visible type.
It also allows to pass the {CompilationState} in a unique_ptr
without a custom deleter.

R=ahaas@chromium.org, mstarzinger@chromium.org

Bug: v8:8238
Change-Id: I5e842723270bc6bb36b605253e3e88103caec61a
Reviewed-on: https://chromium-review.googlesource.com/c/1297956
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56996}