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

This patch removes llvm::is_trivially_{copy/move}_constructible in
favor of std::is_trivially_{copy/move}_constructible.

The previous attempt to remove them in Dec 2020,
c8d406c9, broke builds with "some
versions of GCC" according to
6cd9608f.

It's been 20 months since then, and the minimum requirement for GCC
has been updated to 7.1 from 5.1.

FWIW, I was able to build llvm with gcc 8.4.0.

Differential Revision: https://reviews.llvm.org/D132311