[sanitizer_common] Added 64-bit signed flag parser.

Summary: Adds a 64-bit signed flag parser for GWP-ASan's use.

Reviewers: vlad.tsyrklevich, eugenis

Subscribers: kubamracek, #sanitizers, llvm-commits, vitalybuka, morehouse, pcc, kcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61342

llvm-svn: 359631