[clang] [Clang] [C++26] Implement P2573R2: `= delete("should have a reason");` (PR #86526)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,48 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,pre26 -pedantic %s
+// RUN: %clang_cc1 -std=c++2c -fsyntax-only -verify=expected,compat -Wpre-c++26-compat %s
+// RUN: %clang_cc1 -std=c++2c -fsyntax-only -verify %s
+
+struct S {
+  void a() = delete;
+  void b() = delete(; // expected-error {{expected string literal}} expected-error {{expected ')'}} expected-note {{to match this '('}}
+  void c() = delete(); // expected-error {{expected string literal}}
+  void d() = delete(42); // expected-error {{expected string literal}}
+  void e() = delete("foo"[0]); // expected-error {{expected ')'}} expected-note {{to match this '('}} // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+  void f() = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+
+  S() = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+  ~S() = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+  S(const S&) = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+  S(S&&) = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+  S& operator=(const S&) = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
+  S& operator=(S&&) = delete("foo"); // pre26-warning {{'= delete' with a message is a C++2c extension}} compat-warning {{'= delete' with a message is incompatible with C++ standards before C++2c}}
----------------
AaronBallman wrote:

Another test that probably makes sense because I can see people trying it and getting sad:
```
constexpr const char *getMsg() { return "this is a message"; }
void func() = delete(getMsg());
```


https://github.com/llvm/llvm-project/pull/86526