Revert "[llvm] Remove llvm::is_trivially_{copy/move}_constructible (NFC)"

This reverts commit 01ffe31c.

A build breakage with GCC 7.3 has been reported:

https://reviews.llvm.org/D132311#3797053

FWIW, GCC 7.5 is OK according to Pavel Chupin.  I also personally
tested GCC 8.4.0.