8373289: Test tools/javac/patterns/Exhaustiveness.java timed out then passed

[lahodaj]

The test/langtools/tools/javac/patterns/Exhaustiveness.java contains some degree of randomness (it will generate a big switch, and randomly remove a case from the switch, observing the outcomes). But, both generating the big switch, and the exhaustiveness computation process in javac as such are currently not deterministic, which leads to problems with reproducing any timeout/failure of the test.

The intent in this PR is to ensure the exhaustiveness computation is deterministic, by using LinkedHashSet and LinkedHashMap.

With this change, if the test fails, there will be a better chance on reproducing the failure.

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Issue

• JDK-8373289: Test tools/javac/patterns/Exhaustiveness.java timed out then passed (Bug - P4)

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[mlbridge]

Webrevs

• 00: Full (0c1add14)

[sendaoYan]

Do we need @Override for this method