[libc++] Work around C1XX bug which breaks poisoned hash tests.

Summary: This is my attempt to work around the C1XX bug described to me by @BillyONeal.

Reviewers: BillyONeal, STL_MSFT, CaseyCarter

Reviewed By: BillyONeal

Subscribers: cfe-commits, BillyONeal

Differential Revision: https://reviews.llvm.org/D31260

llvm-svn: 298554