[GWP-ASan] Core Guarded Pool Allocator [4].

[GWP-ASan] Core Guarded Pool Allocator [4].

Summary:
See D60593 for further information.

This patch introduces the core of GWP-ASan, being the guarded pool allocator. This class contains the logic for creating and maintaining allocations in the guarded pool. Its public interface is to be utilised by supporting allocators in order to provide sampled guarded allocation behaviour.

This patch also contains basic functionality tests of the allocator as unittests. The error-catching behaviour will be tested in upcoming patches that use Scudo as an implementing allocator.

Reviewers: vlad.tsyrklevich, eugenis, jfb

Reviewed By: vlad.tsyrklevich

Subscribers: dexonsmith, kubamracek, mgorny, cryptoad, jfb, #sanitizers, llvm-commits, morehouse

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D62872

llvm-svn: 362636