libc++-4.0rc2: Use substituteInPlace instead of the Darwin patch.

On 3.9 the substituteInPlace is equivalent to the patch,
hopefully this is a slightly more robust way to make
the same substitution for the 4.0 version.

Apply this change unconditionally for consistency across versions,
even if the changed strings are unused on other platforms.

Discussion:
https://github.com/NixOS/nixpkgs/pull/22970#discussion_r101926144