After introducing the new pointer-containing Object class in V8 (see
https://docs.google.com/document/d/1_w49sakC1XM1OptjTurBDqO86NE16FH8LwbeUAtrbCo/edit),
gcmole stopped finding errorneous usage of raw pointers in functions that could
trigger GC. This CL modifies the heuristics of the tool to classify Object and
MaybeObject instances as raw pointers, thus giving back the missing warnings.

Updated the gcmole implementation to support modern llvm (tested with llvm 8.0)
for which additional support for MaterializeTemporaryExpr, ExprWithCleanups and
UnaryExprOrTypeTraitExpr was needed.

Basic tests are added to make it harder to introduce such errors without
noticing in the future.

This version gives a lot of false positives when ran on the whole project, see
https://docs.google.com/document/d/1K7eJ0f6m9QX6FZIjZnt_GFtUsjEOC_LpiAwZbcAA3f8/edit

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

Bug: v8:8813
Change-Id: Ic0190a4bc2642eda8880d9f7b30b5145a76a7d89
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1494754
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60099}