[clang] [clang] move -Wcast-function-type under -Wextra (PR #77178)

[Aaron Ballman via cfe-commits]

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@@ -164,6 +164,8 @@ Deprecated Compiler Flags
 Modified Compiler Flags
 -----------------------
 
+* ``-Wextra`` group flag now contains an additional flag i.e, ``-Wcast-function-type``.
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AaronBallman wrote:

```suggestion
- Added ``-Wcast-function-type`` as a warning enabled by ``-Wextra``. #GH76872
```

https://github.com/llvm/llvm-project/pull/77178