[GWP-ASan] Configuration options [3].

Summary:
See D60593 for further information.

This patch introduces the configuration options for GWP-ASan. In general, we expect the supporting allocator to populate the options struct, and give that to GWP-ASan during initialisation. For allocators that are okay with pulling in sanitizer_common, we also provide an optional parser that populates the gwp_asan::Options struct with values provided in the GWP_ASAN_OPTIONS environment variable.

This patch contains very little logic, and all of the testable components (i.e. the optional parser's internal logic) is tested as part of the sanitizer_common testbed.

Reviewers: vlad.tsyrklevich, morehouse, jfb

Reviewed By: morehouse

Subscribers: dexonsmith, kubamracek, mgorny, #sanitizers, llvm-commits, vitalybuka

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D62698

llvm-svn: 362527