Make it possible for RecognitionException to release the parser and

token stream that triggered the error.

These are useful for error diagnostics, but if client code wants to throw
the RecognitionException but discard the parser and token stream, then
the fields in RecognitionException need to be cleared.

This adds RecognitionException.{clearRecognizer,clearInputStream} so that
client code can clear those fields if desired.  It also makes
RecognitionException.ctx weak, so it will go nil at the same time as
the parser is discarded.

runtime/Swift/Sources/Antlr4/RecognitionException.swift

@@ -18,9 +18,9 @@ public class RecognitionException<T:ATNSimulator>  {
     private final var recognizer: Recognizer<T>?
     //Recognizer<AnyObject,ATNSimulator>? ;
 
-    private final let ctx: RuleContext?
+    private final weak var ctx: RuleContext?
 
-    private final let input: IntStream
+    private final var input: IntStream?
 
     /// 
     /// The current _org.antlr.v4.runtime.Token_ when an error occurred. Since not all streams
@@ -113,10 +113,13 @@ public class RecognitionException<T:ATNSimulator>  {
     /// where this exception was thrown, or `null` if the stream is not
     /// available.
     /// 
-    public func getInputStream() -> IntStream {
+    public func getInputStream() -> IntStream? {
         return input
     }
 
+    public func clearInputStream() {
+        input = nil
+    }
 
     public func getOffendingToken() -> Token {
         return offendingToken
@@ -137,4 +140,8 @@ public class RecognitionException<T:ATNSimulator>  {
     public func getRecognizer() -> Recognizer<T>? {
         return recognizer
     }
+
+    public func clearRecognizer() {
+        self.recognizer = nil
+    }
 }