Reconsider CapturingContext

In #4251 I'm introducing a class `CapturingContext` that aims at solving the following architectural dilemma:
* All locations interested in supertypes of a PTB with wildcards need to apply capture (JLS 4.10.2.).
* Interested methods like isCompatibleWith() and findSupertypeOriginatingFrom() etc. do not have the necessary data (scope & source positions) for invoking capture()

A previous partial solution existed in `InferenceContext18.maybeCapture()` (since #3910). Prior to that PR we failed to properly apply JLS 4.10.2 for wildcard-parameterized types.

This issue here will allow us to discuss the problem in more leisure, asking questions like:
* If we keep this strategy: do we have better / more central locations that should invoke enter/leave? Or is it OK to lazily add more enter/leave calls when needed?
* Would we be better off with computing captures more eagerly (stored in the PTB) while necessary context may still be available (is it?) ?