[dataflow] Disallow implicit copy of Environment, use fork() instead

Environments are heavyweight, and copies are observably different from the
original: they introduce new SAT variables, which degrade performance &
debugging. Copies should only be done deliberately, where justified.

Empirically there are several places in the framework where we perform dubious
copies, sometimes entirely accidentally. (see e.g. D153491). Making these
explicit makes this mistake harder.

This patch forces copies to go through fork(), the copy-constructor is private.
This requires changes to existing callsites: some are correct and call fork(),
some are incorrect and are fixed, others are difficult and I left a FIXME.

Differential Revision: https://reviews.llvm.org/D153674