Baseline bump to MSVC 2015 Update 3

[Stephan Bergmann]

As has been brewing for some time and just got accepted in today's ESC 
meeting, we'll bump master (towards LO 5.4) to MSVC 2015 Update 3 as 
basline for MSVC builds on Windows.  That will happen as soon as 
<https://gerrit.libreoffice.org/#/c/22588/> "Drop support for MSVC 2013" 
builds green and is pushed.  Thanks to David for working on that!

* Older branches (libreoffice-5-2, libreoffice-5-3) remain on their MSVC 
2013 baselines.

* The wiki developer documentation will likely need updating.  Any 
volunteers?  (For open source work, a free-of-charge version of MSVC 
2015 Update 3 should be available from Microsoft on the web.)

* Noteworthy improvements this brings to us regarding C++11-and-beyond 
support is that HAVE_CXX11_CONSTEXPR and HAVE_UTF16_STRING_LITERAL 
should now always be true.