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
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