This changes the interaction between {NativeModule} and
{WasmCodeAllocator}. The {WasmCodeAllocator} is a field of
{NativeModule}, and only called directly by the {NativeModule}. So far,
there were two mutexes involved, the {allocation_mutex_} in
{NativeModule}, and {mutex_} in {WasmCodeAllocator}. This caused
problems with lock order inversion.

This CL thus merges the two mutex, by always locking the mutex in
{NativeModule} when calling a non-atomic method in {WasmCodeAllocator}.
This serializes slightly more code, but none of this should be
performance-critical.

This removes the awkward {OptionalLock} class and adds the "Locked"
suffix to a few methods to document that those can only be called
while holding the allocation mutex.

R=jkummerow@chromium.org
CC=​dlehmann@google.com

Bug: v8:11663
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux_arm64_gc_stress_dbg_ng
Cq-Include-Trybots: luci.v8.try:v8_linux_gc_stress_dbg_ng
Change-Id: I8895d61fef23a57b218e068532375bac941a5a77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2831477
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74026}