[scudo] Add Chunk::getSize, rework Chunk::getUsableSize

Summary:
Using `getActuallyAllocatedSize` from the Combined resulting in mediocre
compiled code, as the `ClassId != 0` predicament was not propagated there,
resulting in additional branches and dead code. Move the logic in the frontend,
which results in better compiled code. Also I think it makes it slightly easier
to distinguish between the size the user requested, and the size that was
actually allocated by the allocator.

`const` a couple of things as well.

This has no functional impact.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D44444

llvm-svn: 327525