[GWP-ASan] Add public-facing documentation [6].

[GWP-ASan] Add public-facing documentation [6].

Summary:
Note: Do not submit this documentation until Scudo support is reviewed and submitted (should be #[5]).

See D60593 for further information.

This patch introduces the public-facing documentation for GWP-ASan, as well as updating the definition of one of the options, which wasn't properly merged. The document describes the design and features of GWP-ASan, as well as how to use GWP-ASan from both a user's standpoint, and development documentation for supporting allocators.

Reviewers: jfb, morehouse, vlad.tsyrklevich

Reviewed By: morehouse, vlad.tsyrklevich

Subscribers: kcc, dexonsmith, kubamracek, cryptoad, jfb, #sanitizers, llvm-commits, vlad.tsyrklevich, morehouse

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D62875

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369552 91177308-0d34-0410-b5e6-96231b3b80d8