Add support for guard clauses in Java 21 switch expressions

[TheCK]

This PR adds support for switch statements where a case has a guard clause.

See Issue #983

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[cushon]

Thanks for the fix!

Could you add a test? There are some examples of tests that are only run on newer Java versions here:

google-java-format/core/src/test/java/com/google/googlejavaformat/java/FormatterIntegrationTest.java

Line 49 in 205b85f

private static final ImmutableMultimap<Integer, String> VERSIONED_TESTS =

[cushon]

Since this block is the only part that needs to be different between 17 and 21, I would like to avoid duplicating the rest of the method.

An alternative would be to do something like this:

google-java-format/core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java

Line 3781 in 205b85f

protected List<? extends Tree> getPermitsClause(ClassTree node) {

There could be a getGuard method that returns null in the base class, and Java21InputAstVisitor could override it

[cushon]

It might be worth creating a shared helper to reflectively instantiate the visitor at this point:

private static JavaInputAstVisitor createVisitor(String className) { .. }

[TheCK]

@cushon I've implemented your requested changes. Hope this works for you. If you think it needs more attention still, let me know.