[GWP-ASan] Integration with Scudo [5].

Summary:
See D60593 for further information.

This patch adds GWP-ASan support to the Scudo hardened allocator. It also
implements end-to-end integration tests using Scudo as the backing allocator.
The tests include crash handling for buffer over/underflow as well as
use-after-free detection.

Reviewers: vlad.tsyrklevich, cryptoad

Reviewed By: vlad.tsyrklevich, cryptoad

Subscribers: kubamracek, mgorny, #sanitizers, llvm-commits, morehouse

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D62929

llvm-svn: 363584