[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}}
+}
+
+int test2() { 
+    return; // expected-warning{{non-void function 'test2' should return a value}}
+} 
+
+int test3() { 
+    // no-warning
+} 
+
+int test4() {
+    (void)(bar() || foo()); // no-warning
+} 
+
+void test5() {
+} // no-warning
+
+int test6() {
+  return 0; // no-warning
+}
----------------
AaronBallman wrote:

Some other test cases to add:
```
int test7(void) {
  foo(); // no warning
}

int test8(void) {
  bar(); // warning: non-void function does not return a value
}
```

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