RFR: Add inference support for nullable types

[Maurizio Cimadamore]

This PR adds basic support for inference of generic method calls with nullabe types.

To do that, we enhance both `lub` and `glb` to take nullness markers into account.

Some tweaks were required to make the inference machinery *not* discard seemingly equivalent types such as `String!` and `String?`.

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Commit messages:
 - Merge branch 'lw5' into inference_nullable
 - Add inference support
 - Add null warning test

Changes: https://git.openjdk.org/valhalla/pull/1221/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1221&range=00
  Stats: 102 lines in 6 files changed: 88 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/valhalla/pull/1221.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1221/head:pull/1221

PR: https://git.openjdk.org/valhalla/pull/1221