[PM] I have been educated by several folks that MSVC will never
synthesize a move constructor. Thus, for any types where move semantics
are important (yea, that's essentially every type...) you must
explicitly define the special members. Do so systematically throughout
the pass manager as the core of the design relies heavily on move
semantics.

This will hopefully fix the build with MSVC 2013. We still don't know
why MSVC 2012 accepted this code, but it almost certainly wasn't doing
the right thing.

I've also added explicit to a few single-argument constructors spotted
in passing.