This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] where
we do not need to move the position of [[maybe_unused]] within
declarations.
Notes:
- [[maybe_unused]] is a standard feature of C++17.
- The compiler is far more lenient about the placement of
__attribute__((unused)) than that of [[maybe_unused]].
I'll follow up with another patch to finish up the rest.
358513f662