[llvm] [NFC][CodingStandard] Prefer ``[[maybe_unused]]`` over C-style void cast for unused variables (PR #142850)

[Rahul Joshi via llvm-commits]

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@@ -589,8 +589,10 @@ Prefer C++-style casts
 When casting, use ``static_cast``, ``reinterpret_cast``, and ``const_cast``,
 rather than C-style casts. There are two exceptions to this:
 
-* When casting to ``void`` to suppress warnings about unused variables (as an
-  alternative to ``[[maybe_unused]]``). Prefer C-style casts in this instance.
+* Although ``[[maybe_unused]]`` is preferred to suppress warnings about unused
+  variables in simple cases (for example, variables used only in asserts) casting
+  to ``void`` using a C-style cast can also be used to suppress such warnings if
+  using ``[[maybe_unused]]`` is not acceptable.
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jurahul wrote:

Right, but based on the forum discussion, not everyone is on board with that. I have amended the PR now with the above changes. The delta now is essentially that for variables used only in asserts, we require the use of ``maybe_unused`` over C-style casts. I am hoping that will be an acceptable delta.

https://github.com/llvm/llvm-project/pull/142850