Use std::is_trivial instead of is_trivially_copyable.
Use std::is_trivial instead of is_trivially_copyable.

The oldest versions of GCC we support (before 5) didn't support that
trait. is_trivial is stronger superset that clang::Token fulfills, so
just use that instead.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@314391 91177308-0d34-0410-b5e6-96231b3b80d8

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