SI-6221 inference with Function1 subtypes.

There appears to be no upper bound on the number of places
we have to remove calls to typeSymbol and introduce calls to
baseType. This one was type inference for function parameter
types: worked when expected type was A => B, but not if there
was an implicit conversion from A => B to the expected type.
Merge pull request #2650 from paulp/issue/6221

SI-6221 inference with Function1 subtypes.