This is required to use std::filesystem on macOS as support for it only
landed in the libc++ dylib shipped with 10.15.
See also: https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes
Clang now supports the C++17 <filesystem> library for iOS 13, macOS 10.15, watchOS 6, and tvOS 13.
This is a backport of core#22993