[ASTPrinter] Don't transform type if current type can't have members

Since 9ba892c5af we always transform `CurrentType` in `ASTPrinter` to be an interface type.

This causes issues for variables that whose type is a protocol. Previously, when printing the type, we had `CurrentType` set to an `OpenedArchetypeType`. Now we replace the archetype by a `GenericTypeParamType`, which may not have members, so we are hitting an assertion in `ASTPrinter.cpp:270`.
To resolve this, replace any `OpenedArchetypeType`s with their protocol type before calling `mapTypeOutOfContext`.

Resolves rdar://76580851 [SR-14479]

lib/AST/ASTPrinter.cpp

@@ -923,6 +923,18 @@ public:
     if (Options.TransformContext) {
       Type CurrentType = Options.TransformContext->getBaseType();
       if (CurrentType && CurrentType->hasArchetype()) {
+        // OpenedArchetypeTypes get replaced by a GenericTypeParamType without a
+        // name in mapTypeOutOfContext. The GenericTypeParamType has no children
+        // so we can't use it for TypeTransformContext.
+        // To work around this, replace the OpenedArchetypeType with the type of
+        // the protocol itself.
+        CurrentType = CurrentType.transform([](Type T) -> Type {
+          if (auto *Opened = T->getAs<OpenedArchetypeType>()) {
+            return Opened->getOpenedExistentialType();
+          } else {
+            return T;
+          }
+        });
         CurrentType = CurrentType->mapTypeOutOfContext();
       }
       setCurrentType(CurrentType);

test/SourceKit/CursorInfo/cursor_info_existential_var.swift

@@ -0,0 +1,13 @@
+protocol MyError {
+    var myVar: String { get }
+}
+
+func foo(error: MyError) {
+    _ = error.myVar
+}
+
+// RUN: %sourcekitd-test -req=cursor -pos=6:15 %s -- %s | %FileCheck %s
+// CHECK: myVar
+// CHECK-NEXT: s:27cursor_info_existential_var7MyErrorP5myVarSSvp
+// CHECK-NEXT: source.lang.swift
+// CHECK-NEXT: String