[PM] Make DominatorTrees (corectly) movable so that we can move them

into the new pass manager's analysis cache which stores results
by-value.

Technically speaking, the dom trees were originally not movable but
copyable! This, unsurprisingly, didn't work at all -- the copy was
shallow and just resulted in rampant memory corruption. This change
explicitly forbids copying (as it would need to be a deep copy) and
makes them explicitly movable with the unsurprising boiler plate to
member-wise move them because we can't rely on MSVC to generate this
code for us. =/

llvm-svn: 225966