52985: better cycle error for existential types

- Original cycle error diagnostics PR'd against this issue caught
panic-causing error while resolving std::mem::transmute calls
  - Now, catch invalid use case of not providing a concrete sized type
behind existential type in definining use case.
  - Update relevant test to reflect this new error

52985: revert normalize query changes
      - PR 53588 invalidates 53316, causing a correct cycle error to occur
    with a good span.
      - Don't need to revert the whole merge as the test files are
    still fine, just need to revert the normalize query changes.
      - It should now be correct that infinite recursion detected during
    normalize query type folding is a bug, should have been caught earlier
    (when resolving the existential type's defining use cases).

52985: code review impl
  - Only cause cycle error if anonymous type resolves to anonymous type
that has the same def id (is the same type) as the original (parent)
type.
  - Add test case to cover this case for existential types.

52985: remove Ty prefix from TyAnon
  - To align with changes per commit 6f637da50c

src/test/ui/existential_types/nested_existential_types.rs

@@ -0,0 +1,31 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(existential_type)]
+// compile-pass
+mod my_mod {
+  use std::fmt::Debug;
+
+  pub existential type Foo: Debug;
+  pub existential type Foot: Debug;
+
+  pub fn get_foo() -> Foo {
+      5i32
+  }
+
+  pub fn get_foot() -> Foot {
+      get_foo()
+  }
+}
+
+fn main() {
+    let _: my_mod::Foot = my_mod::get_foot();
+}
+

src/test/ui/existential_types/no_inferrable_concrete_type.stderr

@@ -1,10 +1,15 @@
-error[E0391]: cycle detected when normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: Foo }`
+error[E0391]: cycle detected when processing `Foo`
   --> $DIR/no_inferrable_concrete_type.rs:16:1
    |
 LL | existential type Foo: Copy; //~ cycle detected
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which again requires normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: Foo }`, completing the cycle
+note: ...which requires processing `bar`...
+  --> $DIR/no_inferrable_concrete_type.rs:19:23
+   |
+LL | fn bar(x: Foo) -> Foo { x }
+   |                       ^^^^^
+   = note: ...which again requires processing `Foo`, completing the cycle
 
 error: aborting due to previous error