Be less strict about "All conditional function variants must have identical signatures"

[gvanrossum]

This is one of the followups on #649. Example code:

try:
    from zlip import decompress
except ImportError:
    def decompress(val):
        raise RuntimeError

This produces "error: All conditional function variants must have identical signatures" on the def line. It would be nice if this were more lenient (e.g. computing the intersection of the imported definition and the given def).

[gvanrossum]

Duplicate of #591.