[GWP-ASan] Add names to anonymous mappings.

Summary:
Adds names to anonymous GWP-ASan mappings. This helps Android with debugging
via. /proc/maps, as GWP-ASan-allocated mappings are now easily identifyable.

Reviewers: eugenis, cferris

Reviewed By: eugenis

Subscribers: merge_guards_bot, #sanitizers, llvm-commits, cryptoad, pcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73373