[flang] Improve API for runtime allocator (I/O runtime work part 3)

New<A> used to return an A&; now it returns an OwningPtr<A>
to force better ownership tracking of allocations.  Its API
has also been split into New<A> and SizedNew<A> to allow
allocations with a size override.

Reviewed By: tskeith

Differential Revision: https://reviews.llvm.org/D83108