table, introduce a `FreeRegionMap` data structure. regionck computes the
`FreeRegionMap` for each fn and stores the result into the tcx so that
borrowck can use it (this could perhaps be refactored to have borrowck
recompute the map, but it's a bid tedious to recompute due to the
interaction of closures and free fns). The main reason to do this is
because of rust-lang#22779 -- using a global table was incorrect because when
validating impl method signatures, we want to use the free region
relationships from the *trait*, not the impl.

Fixes rust-lang#22779.