[libc++] Tolerate presence of __deallocate macro

Summary:
On Windows the identifier `__deallocate` is defined as a macro by one of the Windows system headers. Previously libc++ worked around this by `#undef __deallocate` and generating a warning. However this causes the WIN32 version of `__threading_support` to always generate a warning on Windows. This is not OK.

This patch renames all usages of `__deallocate` internally as to not conflict with the macro.

Reviewers: mclow.lists, majnemer, rnk, rsmith, smeenai, compnerd

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28426

llvm-svn: 291332