[scudo] Use our own combined allocator

Summary:
The reasoning behind this change is twofold:
- the current combined allocator (sanitizer_allocator_combined.h) implements
  features that are not relevant for Scudo, making some code redundant, and
  some restrictions not pertinent (alignments for example). This forced us to
  do some weird things between the frontend and our secondary to make things
  work;
- we have enough information to be able to know if a chunk will be serviced by
  the Primary or Secondary, allowing us to avoid extraneous calls to functions
  such as `PointerIsMine` or `CanAllocate`.

As a result, the new scudo-specific combined allocator is very straightforward,
and allows us to remove some now unnecessary code both in the frontend and the
secondary. Unused functions have been left in as unimplemented for now.

It turns out to also be a sizeable performance gain (3% faster in some Android
memory_replay benchmarks, doing some more on other platforms).

Reviewers: alekseyshl, kcc, dvyukov

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33007

llvm-svn: 302830