Unify IfFunction and IfExpression in the grammar

Currently, we have two rules (and therefore two metamodel classes) for the builtins if-function and if-expression:

IfExpression:
  true_ = IfExpressionParameter
  'if' expr = BooleanExpression
  'else' false_ = IfExpressionParameter
;

IfFunction:
  'if' '('
    expr = BooleanExpression ','
    true_ = Parameter ','
    false_ = Parameter
  ')'
;

It should be possible to unify them syntactically in one rule because they are semantically equivalent (the latter leads to identical IfFunction and IfExpression metamodel classes). Something like this should work:

IfFunction:
  (
    true_ = IfExpressionParameter
    'if' expr = BooleanExpression
    'else' false_ = IfExpressionParameter
  ) |
  (
    'if' '('
      expr = BooleanExpression ','
      true_ = Parameter ','
      false_ = Parameter
    ')'
  )
;

Also it should be possible to eliminate the IfExpressionParameter in favor of Parameter.