This fixes a problem where TransformationInlineFunction could lead to
distinct instructions having identical unique ids. It adds a validity
check to detect this problem in general.
Fixes#3911.
TransformationContext now holds a std::unique_ptr to a FactManager,
rather than a plain pointer. This makes it easier for clients of
TransformationContext to work with heap-allocated instances of
TransformationContext, which is needed in some upcoming work.
