MissingCasesInEnumSwitch handles case labels with multiple expressions

[pkoenig10]

MissingCasesInEnumSwitch does not correctly handle multi-expression case labels in switch expressions. It uses the deprecated CaseTree#getExpression method, which only returns the first expression in the case label.

A simple example is shown below:

enum Case { ONE, TWO }
void m(Case c) {
  switch (c) {
    case ONE, TWO -> {}
  }
}

This code will produce the following error:

[MissingCasesInEnumSwitch] Non-exhaustive switch; either add a default or handle the remaining cases: TWO

This PR updates MissingCasesInEnumSwitch to instead use the non-deprecated CaseTree#getExpressions method, which returns all expressions in the case label.

Unfortunately this requires using reflection because the CaseTree#getExpressions method was not added until Java 12.

[pkoenig10]

@cushon Any plans to cut a new release soon? I'm eager to pick up this fix.

See also #1415.

[cushon]

@pkoenig10 I'm planning to do one soon (like in the next week or so): #2061