Fix retain cycle between Parser and ParserATNSimulator.

This was causing the entire parser to be retained, resulting in a large
memory leak.

This fix simply changes the reference from ParserATNSimulator to Parser
to be unowned.

Ditto between Lexer and LexerATNSimulator, except this reference is made
weak because LexerATNSimulator.recog is nullable.  (That difference is
dubious IMHO, but I'm leaving it intact for now.)

runtime/Swift/Sources/Antlr4/atn/LexerATNSimulator.swift

@@ -50,7 +50,7 @@ open class LexerATNSimulator: ATNSimulator {
     }
 
 
-    internal let recog: Lexer?
+    internal weak var recog: Lexer?
 
     /// 
     /// The current token's starting index into the character stream.

runtime/Swift/Sources/Antlr4/atn/ParserATNSimulator.swift

@@ -250,7 +250,8 @@ open class ParserATNSimulator: ATNSimulator {
         }
         return false
     }()
-    internal final var parser: Parser
+
+    internal final unowned let parser: Parser
 
     public final var decisionToDFA: [DFA]