Move constructors out-of-line and flesh out their documentation. No

Move constructors out-of-line and flesh out their documentation. No
functionality changed. This is in preparation for some refactoring of
how this class behaves.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150941 91177308-0d34-0410-b5e6-96231b3b80d8