[DebuggerSupport] Unbreak closures in the expression parser.

The new trick is that of leavign an unresolved identifier for
the expression parser to fill in.

<rdar://problem/47982630>

Author: Davide Italiano

lib/Parse/Lexer.cpp

@@ -953,8 +953,6 @@ 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);
   }
 }

lib/Parse/ParseExpr.cpp

@@ -2915,6 +2915,12 @@ Expr *Parser::parseExprAnonClosureArg() {
   auto closure = dyn_cast_or_null<ClosureExpr>(
       dyn_cast<AbstractClosureExpr>(CurDeclContext));
   if (!closure) {
+    if (Context.LangOpts.DebuggerSupport) {
+      auto refKind = DeclRefKind::Ordinary;
+      auto identifier = Context.getIdentifier(Name);
+      return new (Context) UnresolvedDeclRefExpr(DeclName(identifier), refKind,
+                                                 DeclNameLoc(Loc));
+    }
     diagnose(Loc, diag::anon_closure_arg_not_in_closure);
     return new (Context) ErrorExpr(Loc);
   }