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

[Nick Desaulniers via cfe-commits]

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+// RUN: %clang_cc1 %s -fsyntax-only -Wextra -verify
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nickdesaulniers wrote:

If this diagnostic is not already well tested, adding all of these additional unit tests makes sense.

But it looks like there's quite a number of existing tests in other files.  I don't think we should add a new file just to check this diagnostics behavior via `-Wextra` via a whole new file.

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