[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
----------------
AaronBallman wrote:

When you add the other RUN line, it should use `-verify=return-type`.
```suggestion
// RUN: %clang_cc1 -Wno-return-type -Wreturn-mismatch -fsyntax-only -verify=return-mismatch %s
```

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