RFR: [bworld] Emit lint warnings if overriding method does not have same nullability [v9]

[Vicente Romero]

> emit a lint warning for cases like:
> 
> class Test {
>     String! m(String s) { return ""; }
> }
> class Sub extends Test {
>     @Override
>     String m(String s) { return null; }
> }
> 
> or:
> 
> class Test {
>     String m(String! s) { return ""; }
> }
> class Sub extends Test {
>     @Override
>     String m(String s) { return null; }
> }

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  changes needed after merge

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/2058/files
  - new: https://git.openjdk.org/valhalla/pull/2058/files/ef61c282..585aec6b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=2058&range=08
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=2058&range=07-08

  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/valhalla/pull/2058.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2058/head:pull/2058

PR: https://git.openjdk.org/valhalla/pull/2058