[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() {
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AaronBallman wrote:

```suggestion
void test1(void) {
```
Same change should be made to all the functions in this file.

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