[GWP-ASan] Fixed issue with c++ standard library dependency.
Removed dependency on c++ standard library. Some supporting allocators (namely Scudo on Fuchsia, and shortly, scudo standalone) has a hard requirement of no c++stdlib.
This patch updates the build system so that we don't have any c++ stdlib dependencies. It also will conveniently fix a racy build-order bug discrepency between GWP-ASan and libc++.
Reviewers: phosek, morehouse
Reviewed By: phosek, morehouse
Subscribers: kubamracek, mgorny, cryptoad, #sanitizers, llvm-commits, beanz, smeenai, vitalybuka
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D62048