This refactoring is a big step towards separating Turbofan-related code
from backend independent code. This will allow us to include way less
headers from "src/compiler" at various places.

The {WasmCompilationUnit} contained information for Turbofan
compilation, and for Liftoff compilation. This CL tears this apart, such
that {WasmCompilationUnit} holds backend-independent information, plus
a pointer to either {LiftoffCompilationUnit} or
{TurbofanWasmCompilationUnit}. These pointers are opaque, so that
{function-compiler.h}, defining {WasmCompilationUnit}, does not need to
include any Turbofan specific or Liftoff specific headers.

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

Bug: v8:7570, v8:6600
Change-Id: I024c9a23508ee1b4b3cbe1d068c8e785d732daca
Reviewed-on: https://chromium-review.googlesource.com/1016640
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52684}