RFR: 7903093: Detect invalid class names in CleanAction

[Christian Stein]

Check the validity of all given class names by delegating the check to the `javax.lang.model.SourceVersion#isName` method.

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Commit messages:
 - 7903093: Detect invalid class names in CleanAction

Changes: https://git.openjdk.java.net/jtreg/pull/53/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=53&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903093
  Stats: 4 lines in 1 file changed: 2 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/53.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/53/head:pull/53

PR: https://git.openjdk.java.net/jtreg/pull/53