[libc++] reverts commit a5f5aad568bb7a91ceee47641f3076ac339ef8c7.

The commit being reverted caused segfaults when building
with libc++ and GCC (and possibly other configurations).

Differential Revision: https://reviews.llvm.org/D62868

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369270 91177308-0d34-0410-b5e6-96231b3b80d8