Move constructors should be noexcept

Move constructors should be noexcept

Summary:
The compiler is able to make some nice optimizations when it
knows that move constructors are noexcept. (In particular, it helps a
lot inside of std::vector if you don't need to worry about the
possibility of exception during reallocation.) Some move constructors in
folly are obviously moveable: change them.

Test Plan: unit tests

Reviewed By: delong.j@fb.com

Subscribers: trunkagent, sdwilsh, njormrod, folly-diffs@

FB internal diff: D1644296

Tasks: 5486739

Signature: t1:1644296:1414618605:d9a0db5193c82650b96e9c62b019d5da218b15c5