[scudo] Simplify internal names (NFC)

Summary:
There is currently too much redundancy in the class/variable/* names in Scudo:
- we are in the namespace `__scudo`, so there is no point in having something
  named `ScudoX` to end up with a final name of `__scudo::ScudoX`;
- there are a lot of types/* that have `Allocator` in the name, given that
  Scudo is an allocator I figure this doubles up as well.

So change a bunch of the Scudo names to make them shorter, less redundant, and
overall simpler. They should still be pretty self explaining (or at least it
looks so to me).

The TSD part will be done in another CL (eg `__scudo::ScudoTSD`).

Reviewers: alekseyshl, eugenis

Reviewed By: alekseyshl

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D49505

llvm-svn: 337557