Add a convenience initializer for IfStmt (optionally taking a second trailing builder closure)

[fwcd]

As proposed in #506 (comment), this adds a new convenience initializer for IfStmt making both if and if-else statements nicely expressible using (multiple) trailing builder closures:

IfStmt(conditions: ...) {
  // if-body
}

IfStmt(conditions: ...) {
  // if-body
} elseBody: {
  // else-body
}

[fwcd]

@swift-ci please test

[fwcd]

@swift-ci please test