Chrono: merge timepoint io improvements.

git-svn-id: http://svn.boost.org/svn/boost/branches/release@86491 b8fc166d-592f-0410-95f2-cb63ce0dd405
Chrono: merge timepoint io improvements.

git-svn-id: http://svn.boost.org/svn/boost/branches/release@86492 b8fc166d-592f-0410-95f2-cb63ce0dd405
Merge 86426 from trunk. This should fix msvc test failures.

git-svn-id: http://svn.boost.org/svn/boost/branches/release@86503 b8fc166d-592f-0410-95f2-cb63ce0dd405