[libc++] Allow building with C++17.

After committing D92214 it was noticed libc++ no longer builds with
C++17. For now reenable building with C++17. This is intended to be a
temporary measure in the future a C++20 capable compiler will be
required.

GitOrigin-RevId: 8869e2f969101426e8615bb3cb0e7186938c23c7
1 file changed