[libcxxabi]: Resync llvm & libcxxabi's demangler

Sadly the demangler copies have diverged.  This resyncs them by
a) pulling the meaningful llvm changes into libcxxabi's gold copy.
b) rerunning the sync script.

I notice uses of placement new, which assume the allocator succeeds --
that's incorrect in general, but an orthogonal problem.

Reviewed By: bader

Differential Revision: https://reviews.llvm.org/D135446