Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r261283:

Import libc++ 3.4 release. This contains a lot of bugfixes, and some
preliminary support for C++1y.

MFC r261604:

HEAD is not buildable for the past day. Commit a 'quick fix' in order to permit
buildworld to complete.

Reviewed by: theraven

MFC r261608:

Apply a cleaner solution for the sign warnings that can occur when
compiling libc++'s <locale> header with -Wsystem-headers on.

This has also been submitted upstream.

Reported by: asomers

MFC r261801:

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 r246487 (by theraven):

Import new libc++ to head. Various small fixes and cleanups.

MFC r249998:

Merge libc++ trunk r180598. Contains several minor cleanups and bug
fixes, no major changes.

Merged libcxxrt and libc++. Now available for testing on 9-stable with
-stdlib=libc++. Changes to libstdc++ not yet merged, so it is not yet possible
to mix libstdc++ and libc++ in the same program.

Merged revisions: 226702,226785,227006,227755,227983,227987,228531,228630,228761,229067,230127,232950,233098,234715-234716,234772

Import new versions of libcxxrt and libc++.
Please tests any C++ code you care about with -stdlib=libc++!

Approved by: dim (mentor)

Import libc++ / libcxxrt into base. Not build by default yet (use
MK_LIBCPLUSPLUS=yes to enable). This is a work-in-progress. It works for
me, but is not guaranteed to work for anyone else and may eat your dog.

To build C++ using libc++, add -stdlib=libc++ to your CXX and LD flags.

Bug reports welcome, bug fixes even more welcome...

Approved by: dim (mentor)

Import libcxxrt / libc++ into a vendor branch.

Approved by: dim (mentor)