RFR: Add option to disable parametric nullness by default

[Maurizio Cimadamore]

This PR adds an option to disable parametric nullness by default, namely `-XDtvarUnspecifiedNullity`.

This option alters the behavior of the various nullness predicates (which have now been moved to `Types`).

Now we consult parametric nullity in two places:
* when we compute the type of some generic class member (`Types::memberType`)
* when we collect bounds for inference variables.

If parameteric nullness is disabled, we simply drop nullness markers on the floor.

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Commit messages:
 - Add support to disable parametric nullity by default
 - Merge branch 'lw5' into parametric_null
 - Merge branch 'lw5' into inference_nullable
 - Add inference support
 - Add null warning test

Changes: https://git.openjdk.org/valhalla/pull/1222/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1222&range=00
  Stats: 157 lines in 15 files changed: 85 ins; 36 del; 36 mod
  Patch: https://git.openjdk.org/valhalla/pull/1222.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1222/head:pull/1222

PR: https://git.openjdk.org/valhalla/pull/1222