[Swift] invalid swift generated for Dynamically-Scoped Attributes

when using the (slightly altered for swift) grammar from the [doc](https://github.com/antlr/antlr4/blob/master/doc/actions.md#dynamically-scoped-attributes)

```
grammar DynScope;

prog: block ;

block
/* List of symbols defined within this block */
locals [
symbols: [String]
]
: '{' decl* stat+ '}'
// print out all symbols found in block
// $block::symbols evaluates to a List as defined in scope
{print("symbols=\($symbols)");}
;

/** Match a declaration and add identifier name to list of symbols */
decl: 'int' ID {$block::symbols.append($ID.text);} ';' ;

/** Match an assignment then test list of symbols to verify
* that it contains the variable on the left side of the assignment.
* Method contains() is List.contains() because $block::symbols
* is a List.
*/
stat: ID '=' INT ';'
{
if ( !$block::symbols.contains($ID.text) ) {
print("undefined variable: "+$ID.text);
}
}
| block
;

ID : [a-z]+ ;
INT : [0-9]+ ;
WS : [ \t\r\n]+ -> skip ;

```

the generated code is:
`(BlockContext)getInvokingContext(1)).symbols.append((_localctx.castdown(DeclContext.self)._ID != nil ? _localctx.castdown(DeclContext.self)._ID!.getText()! : ""));`
which uses an invalid c-style cast.

it should generate something like:
`(getInvokingContext(1) as! BlockContext).symbols.append((_localctx.castdown(DeclContext.self)._ID != nil ? _localctx.castdown(DeclContext.self)._ID!.getText()! : ""));`

Start with the dynamically scoped attributes example in doc/actions.md and reproduce the grammar using the Swift target. Trace the generated output containing `(BlockContext)getInvokingContext(1)` and verify that the completed output uses valid Swift casting while preserving the surrounding expression.

Written by the indexing model from the issue text.