RFR: 8305671: javac rejects semicolons in compilation units with no imports [v4]

[Archie Cobbs]

> The fix for [JDK-8027682](https://bugs.openjdk.org/browse/JDK-8027682) created a new bug in which a program like this:
> 
> package p;;
> 
> would be rejected.
> 
> This is due to the previous fix swallowing all the semicolons. Instead, in the case of semicolons followed by EOF, it needs to swallow all but the last one.

Archie Cobbs 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 five additional commits since the last revision:

 - Merge branch 'master' into JDK-8305671
 - Consume extra semicolons in a more straightforward way.
 - Merge branch 'master' into JDK-8305671
 - Use ToolBox and add more tests per review suggestion.
 - Allow a package declaration followed (only) by multiple commas.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13361/files
  - new: https://git.openjdk.org/jdk/pull/13361/files/f2de70cf..0561a6cd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13361&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13361&range=02-03

  Stats: 19380 lines in 404 files changed: 13989 ins; 3988 del; 1403 mod
  Patch: https://git.openjdk.org/jdk/pull/13361.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13361/head:pull/13361

PR: https://git.openjdk.org/jdk/pull/13361