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

[Aaron Ballman via cfe-commits]

================
@@ -94,6 +94,7 @@ Deprecated Compiler Flags
 
 Modified Compiler Flags
 -----------------------
+- The `-Wreturn-type` flag has been modified to split some of its functionality into `-Wreturn-mismatch` flag.
----------------
AaronBallman wrote:

```suggestion
- Added a new diagnostic flag ``-Wreturn-mismatch`` which is grouped under
``-Wreturn-type``, and moved some of the diagnostics previously controlled by
``-Wreturn-type`` under this new flag. Fixes #GH72116.
```

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