[Lexer] Allow '$x' to be parsed as identifier in debugger mode.

This is needed to support `print $0` in lldb where `$0` is the
shortcut for a closure argument. The associated lldb change
will be merged together with this one and will contain a test.

<rdar://problem/201719448>

Author: Davide Italiano

lib/Parse/Lexer.cpp

@@ -936,6 +936,8 @@ void Lexer::lexDollarIdent() {
     // independent of language mode.
     return formToken(tok::identifier, tokStart);
   } else {
+    if (LangOpts.EnableDollarIdentifiers && !LangOpts.Playground)
+      return formToken(tok::identifier, tokStart);
     return formToken(tok::dollarident, tokStart);
   }
 }