tsan: include internal allocator into deadlock detection

Now that we lock the internal allocator around fork,
it's possible it will create additional deadlocks.
Add a fake mutex that substitutes the internal allocator
for the purposes of deadlock detection.

Depends on D114531.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D114532

GitOrigin-RevId: 6f85d68e6ab29e544b301d7ac84de2819b8d4b7a