LLVM contains a helpful function for getting the size of a C-style
array: llvm::array_lengthof. This is useful prior to C++17, but not as
helpful for C++17 or later: std::size already has support for C-style
Change call sites to use std::size instead. Leave the few call sites that
use a locally defined array_lengthof that are meant to test previous bugs
with NTTPs in clang analyzer and SemaTemplate.