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

[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. =/

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225966 91177308-0d34-0410-b5e6-96231b3b80d8