[NFC][CodingStandard] Deprecate use of void casts to suppress warnings

Recommend using attribute `[[maybe_unused]`` for variables that
may be unused in non-assert enabled builds to suppress unused variable
warnings.

Author: jurahul

llvm/docs/CodingStandards.rst

@@ -587,10 +587,7 @@ 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.
+rather than C-style casts. There is one exception to this:
 
 * When casting between integral types (including enums that are not strongly-
   typed), functional-style casts are permitted as an alternative to
@@ -1286,17 +1283,17 @@ value" warning when assertions are disabled.  For example, this code will warn:
 
 These are two interesting different cases. In the first case, the call to
 ``V.size()`` is only useful for the assert, and we don't want it executed when
-assertions are disabled.  Code like this should move the call into the assert
+assertions are disabled. Code like this should move the call into the assert
 itself.  In the second case, the side effects of the call must happen whether
-the assert is enabled or not.  In this case, the value should be cast to void to
-disable the warning.  To be specific, it is preferred to write the code like
-this:
+the assert is enabled or not. In this case, the value should be defined using
+the ``[[maybe_unused]]`` attribute disable the warning. To be specific, it is
+preferred to write the code like this:
 
 .. code-block:: c++
 
   assert(V.size() > 42 && "Vector smaller than it should be");
 
-  bool NewToSet = Myset.insert(Value); (void)NewToSet;
+  [[maybe_unused]] bool NewToSet = Myset.insert(Value);
   assert(NewToSet && "The value shouldn't be in the set yet");
 
 Do Not Use ``using namespace std``