[clang] [Clang][Sema] Skip checking anonymous enum in using enum declaration (PR #87144)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+// expected-no-diagnostics
+
+enum {A, S, D, F};
+int main() {
+    using asdf = decltype(A);
+    using enum asdf; // this line causes the crash
+    return 0;
+}
----------------
AaronBallman wrote:

I'd like to see an additional test case demonstrating that we correctly added the enumerators to the correct scope:
```
enum {A, S, D, F};
constexpr struct T {
  using asdf = decltype(A);
  using enum asdf;
} t;

static_assert(t.D == D);
static_assert(T::S == S);
```
and
```
enum {A, S, D, F};
constexpr struct T {
  struct {
    using asdf = decltype(A);
    using enum asdf;
  } inner;
} t;

static_assert(t.inner.D == D);
static_assert(t.D == D); // expected-error
```

https://github.com/llvm/llvm-project/pull/87144