Switch from llvm::is_trivially_copyable to std::is_trivially_copyable

GCC<5 did not support std::is_trivially_copyable. Now LLVM builds require 5.1
we can migrate to std::is_trivially_copyable.

The Optional.h change made MSVC choke
(https://buildkite.com/llvm-project/premerge-checks/builds/18587#cd1bb616-ffdc-4581-9795-b42c284196de)
so I leave it out for now.

Differential Revision: https://reviews.llvm.org/D92514