IR: Return unique_ptr from MDNode::getTemporary()

IR: Return unique_ptr from MDNode::getTemporary()

Change `MDTuple::getTemporary()` and `MDLocation::getTemporary()` to
return (effectively) `std::unique_ptr<T, MDNode::deleteTemporary>`, and
clean up call sites.  (For now, `DIBuilder` call sites just call
`release()` immediately.)

There's an accompanying change in each of clang and polly to use the new
API.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226504 91177308-0d34-0410-b5e6-96231b3b80d8