ASTMangler: Fix mangling of invalid dependent member types

Fixes <rdar://problem/48014983>.

Author: slavapestov

lib/AST/ASTMangler.cpp

@@ -961,8 +961,8 @@ void ASTMangler::appendType(Type type) {
         // Dependent members of non-generic-param types are not canonical, but
         // we may still want to mangle them for debugging or indexing purposes.
         appendType(DepTy->getBase());
-        appendAssociatedTypeName(DepTy);
-        appendOperator("qa");
+        appendIdentifier(DepTy->getName().str());
+        appendOperator("Qa");
       }
       addTypeSubstitution(DepTy);
       return;

test/Index/invalid_code.swift

@@ -30,3 +30,10 @@ class CrashTest2 {
     }
   }
 }
+
+public struct BadCollection: Collection {
+    public var startIndex: Index { }
+    public var endIndex: Index { }
+    public func index(after index: Index) -> Index { }
+    public subscript(position: Index) -> Element { }
+}