RFR: type of TypeVar.ref is incorrect and should be fixed [v2]

[Vicente Romero]

> The type of TypeVar.ref is incorrect and should be fixed, in particular due to a bug, the compiler is not able to differentiate between  `T` and `T.ref`. Also did some cleanup to warnings related code, in particular there should be a different warning for cases when a universal tvar is not assigned `null` but it is assigned a value that could be null at run time. This warning should not be the same as the one for a plain null assignment.

Vicente Romero has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'universal-tvars' into fixing.warning.bugs
 - clean up to warnings related code

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Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/658/files
  - new: https://git.openjdk.java.net/valhalla/pull/658/files/b3edd868..c8bb952a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=658&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=658&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/658.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/658/head:pull/658

PR: https://git.openjdk.java.net/valhalla/pull/658