Make OptionalBindingCondition and MatchingPatternCondition expressible as ConditionElement

[fwcd]

As proposed in #506 (comment), this conforms ExpressibleAsOptionalBindingCondition and ExpressibleAsMatchingPatternCondition to ExpressibleAsConditionElement, making it more convenient to express if-let and if-case statements with the DSL:

// if let x = y { ... }
IfStmt(
  conditions: OptionalBindingCondition(
    letOrVarKeyword: .let,
    pattern: "x",
    initializer: InitializerClause(value: "y")
  )
) { ... }

// if case .x = y { ... }
IfStmt(
  conditions: MatchingPatternCondition(
    pattern: ExpressionPattern(expression: MemberAccessExpr(name: "x")),
    initializer: InitializerClause(value: "y")
  )
) { ... }

[fwcd]

@swift-ci please test

[fwcd]

@swift-ci please test