Remove llvm::is_trivially_copyable (NFC)

This patch removes llvm::is_trivially_copyable as it seems to be dead.
Once I remove it, HAVE_STD_IS_TRIVIALLY_COPYABLE has no users, so this
patch removes the macro also.

The comment on llvm::is_trivially_copyable mentions GCC 4.9, but note
that we now require GCC 7.1 or higher.

Differential Revision: https://reviews.llvm.org/D132328