[dataflow] Use consistent, symmetrical, non-mutating erased signature for join()

Mutating join() isn't used and so appears to be an anti-optimization.
Having Lattice vs Environment inconsistent is awkward, particularly when trying
to minimize copies while joining.

This patch eliminates the difference, but doesn't actually change the signature
of join on concrete lattice types (as that's a breaking change).

Differential Revision: https://reviews.llvm.org/D153908