In ms mode, a move assignment operator shouldn't mark a copy ctor as deleted.

MSVC2010's pair class has a move assignment operator but no explicit copy
constructor, which makes it unusable without this change.

For symmetry, let move copy constructors not mark the default assignment
operator as deleted either. Both changes match cl.exe's behavior.  Fixes
pr11826.

Also update the standard excerpt to point to the right paragraph.

llvm-svn: 148675