Add constexpr to pair

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D80558

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 737a4501e815d8dd57e5095dbbbede500dfa8ccb
14 files changed