An ABI incompatibility crept into the libc++ 3.4 import in r261283.  It
was caused by upstream libc++ commit r194536, which aimed to make the
headers more standards-compliant, by making std::pair's copy constructor
trivial.  Unfortunately, this could cause certain C++ applications using
shared libraries built against the previous version of libc++ to crash.

Fix the ABI incompatibility by making std::pair's copy constructor
non-trivial again.

Please note: Any C++ applications or shared libraries built with libc++
between r261283 and this revision should be recompiled.

Reported by:	stefanf
MFC after:	3 weeks
X-MFC-With:	r261283
Add a note to UPDATING about the ABI compatibility fix done in r261801.