Changes to allow propagating type parameters with no inferences.

A summary of the bigger pieces I can think of:

* When strictFunctionTypes is true if there are no inferences for a
type parameter then the type parameter itself will be returned instead
of defaulting to `{}`. After chooseOverload returns a successful
signature, the return type of that signature is checked and if the
return type is a function type with a single call signature then any
free type parameters found in that call signature are converted to type
parameters for the call signature. If the return type is not a call
signature then any free type parameters are converted to `{}` at that
point.

* inferFromSignatures will instantiate the source in the context of the
target if the source signature has type parameters and
strictFunctionTypes is true.

* StrictFunctionTypes is required in both of the above cases because of
errors that pop up otherwise when there are not contravariant inferences.

* Seemingly free type parameters that are outerTypeParameters of the
call being inferred are ignored instead of being erased. This is to
avoid erasing type parameters when calling functions inside the body of
a generic function.

* When choosing an overload with multiple signatures that have
contextually typed arguments, any contextual types that are assigned are
reset when trying a new candidate.

Updated baselines will be committed in the following commit.

Author: kpdonn