Second attempt to fix "not found" error for type vars in bounds

[copybara-service]

Second attempt to fix "not found" error for type vars in bounds

First attempt was #671
and I had to roll it back, since it caused breakages in two ways:

1. Sometimes ParameterElement from type.parameters had enclosingElement
   set to null and we use that in one helper function. That was easy to
   fix, we could just pass methodElement to that function directly.
   It's probably correct that ParameterElement of a FunctionType
   doesn't link back to a MethodElement, but it's weird that sometimes
   it does, so it wasn't caught in the tests. I had to get rid of
   using type.parameters anyway because of the second problem.
2. type.parameters don't contain parameters' default values (totally
   correct, since default values belong to methods, not to types), but
   that means we can't use them, since we do need default values.

So I ended up with a more hacky solution, that uses type.typeFormals
just to get correct references for method's type parameters, and then
uses typeParameters, returnType and parameters for the rest as
before.

Original commit description:

Use FunctionTypedElement.type while generating method overrides

Turns out FunctionTypedElement.typeParameters could be inconsistent
for MethodMembers returned by InheritanceManager3.getMember2.
FunctionTypedElement.type.typeFormals seem to be always good, but
we have to also use type.parameters and type.returnType instead
of just parameters and returnType in this case.

Fixes #658