[scudo] Scudo thread specific data refactor, part 1

Summary:
We are going through an overhaul of Scudo's TSD, to allow for new platforms
to be integrated more easily, and make the code more sound.

This first part is mostly renaming, preferring some shorter names, correcting
some comments. I removed `getPrng` and `getAllocatorCache` to directly access
the members, there was not really any benefit to them (and it was suggested by
Dmitry in D37590).

The only functional change is in `scudo_tls_android.cpp`: we enforce bounds to
the `NumberOfTSDs` and most of the logic in `getTSDAndLockSlow` is skipped if we
only have 1 TSD.

Reviewers: alekseyshl, dvyukov, kcc

Reviewed By: dvyukov

Subscribers: llvm-commits, srhines

Differential Revision: https://reviews.llvm.org/D38139

llvm-svn: 313987