[clang] [clang] Fix static_cast bypassing access control (PR #132285)

[via cfe-commits]

================
@@ -23,3 +22,67 @@ void test(A &a, B &b) {
   const A &&ar10 = static_cast<const A&&>(xvalue<A>());
   const A &&ar11 = static_cast<const A&&>(xvalue<B>());
 }
+
+struct C : private A { // expected-note 4 {{declared private here}}
+    C&& that();
+
+    void f() {
+        (void)static_cast<A&&>(*this);
+        (void)static_cast<const A&&>(*this);
+
+        (void)static_cast<A&&>(that());
+        (void)static_cast<const A&&>(that());
+    }
+};
+C c;
+const C cc;
+
+void f() {
+    static_cast<A&&>(c);        // expected-error {{cannot cast 'C' to its private base class 'A'}}
+    static_cast<A&&>(c.that()); // expected-error {{cannot cast 'C' to its private base class 'A'}}
+
+    static_cast<const A&&>(c);        // expected-error {{cannot cast 'C' to its private base class 'const A'}}
+    static_cast<const A&&>(c.that()); // expected-error {{cannot cast 'C' to its private base class 'const A'}}
+}
+
+constexpr auto v = (
+    (A&&)c,
+    (A&&)(C&&)c,
+    (A&&)cc,
+    (A&&)(const C&&)c,
+    (const A&&)c,
+    (const A&&)(C&&)c,
+    (const A&&)cc,
+    (const A&&)(const C&&)c
+);
----------------
Sirraide wrote:

```suggestion
constexpr bool g() {
    (A&&)c;
    (A&&)(C&&)c;
    (A&&)cc;
    (A&&)(const C&&)c;
    (const A&&)c;
    (const A&&)(C&&)c;
    (const A&&)cc;
    (const A&&)(const C&&)c;
    return true;
}
static_assert(g());
```
This here is a bit gross; I’d probably move all of thse into a function instead (you may have to add more `(void)` cases or alternatively just put `-Wno-unused-value` in the RUN line honestly).

https://github.com/llvm/llvm-project/pull/132285