Issue #6615: Add support for Java 14 switch and yield

This PR will introduce support for Java 14 switch expressions, and yield statements.

Relevant JLS grammar for switch expressions:

UnaryExpression:
PreIncrementExpression
PreDecrementExpression
+ UnaryExpression
- UnaryExpression
UnaryExpressionNotPlusMinus

PreIncrementExpression:
++ UnaryExpression

PreDecrementExpression:
-- UnaryExpression

UnaryExpressionNotPlusMinus:
PostfixExpression
~ UnaryExpression
! UnaryExpression
CastExpression
SwitchExpression

SwitchExpression:
switch ( Expression ) SwitchBlock

SwitchBlock:
{ SwitchLabeledRule {SwitchLabeledRule} }
{ {SwitchLabeledStatementGroup} {SwitchLabel :} }

SwitchLabeledRule:
SwitchLabeledExpression
SwitchLabeledBlock
SwitchLabeledThrowStatement

SwitchLabeledExpression:
SwitchLabel -> Expression ;

SwitchLabeledBlock:
SwitchLabel -> Block

SwitchLabeledThrowStatement:
SwitchLabel -> ThrowStatement

SwitchLabel:
case CaseConstant {, CaseConstant}
default

CaseConstant:
ConditionalExpression

SwitchLabeledStatementGroup:
SwitchLabel : {SwitchLabel :} BlockStatements

Relevant grammar for yield statements:

YieldStatement:
yield Expression ;

I have introduced the new token, SWITCH_RULE, for the following reasons:

1. From JLS: When a selector expression matches a switch label for a switch labeled rule, the labeled expression or statement is executed and nothing else.

   This means that there is no reason to check for fall-through, as in the case of switch case groups. In checks, we can simply conclude that we are in a switch rule, and know that there will be definite assignment and no fall through.

2. The java grammar differentiates between the two, so we should also.

   The body of both a switch statement and a switch expression (15.28) is known as a switch block. This block can consist of either:

   Switch labeled rules, which use -> to introduce either a switch labeled expression, switch labeled block, or a switch labeled throw statement; or

   Switch labeled statement groups, which use : to introduce switch labeled block statements.