[libc++] Remove the ability to use braced-init for filesystem paths

According to my reading of http://eel.is/c++draft/filesystems#fs.class.path,
the Standard doesn't actually mention that this should work. Since other
implementations don't allow it, allowing it in libc++ is just setting a
portability trap.

Supersedes https://reviews.llvm.org/D89865.

Differential Revision: https://reviews.llvm.org/D95975