[clang] Add new flag -Wreturn-mismatch (PR #82872)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -Wno-return-type -Wreturn-mismatch -fsyntax-only -verify %s
+
+// Test that -Wreturn-mismatch is enabled and -Wreturn-type is disabled.
+
+int foo(void) __attribute__((noreturn));
+int bar(void);
+
+void test1() {
+  return 1; // expected-warning{{void function 'test1' should not return a value}}
----------------
AaronBallman wrote:

```suggestion
  return 1; // return-mismatch-warning{{void function 'test1' should not return a value}}
```

https://github.com/llvm/llvm-project/pull/82872