[doc] Add casting style preference to coding standards

Differential Revision: https://reviews.llvm.org/D151187

Author: jh7370

llvm/docs/CodingStandards.rst

@@ -590,6 +590,19 @@ templates like :ref:`isa\<>, cast\<>, and dyn_cast\<> <isa>`.
 This form of RTTI is opt-in and can be
 :doc:`added to any class <HowToSetUpLLVMStyleRTTI>`.
 
+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.
+
+* When casting between integral types (including enums that are not strongly-
+  typed), functional-style casts are permitted as an alternative to
+  ``static_cast``.
+
 .. _static constructor:
 
 Do not use Static Constructors