[libc++] Improve no wide characters configuration.

When wide characters are supported libc++ manually translates a
`narrow non-breaking space` and a `non-breaking space` to a space.
This behaviour wasn't available when wide characters were disabled.
This enables an emulation for that configuration.

Updating the libc++ Docker image to Ubuntu Focal caused some breakage.
This was temporary disabled in D112737. This re-enables four of these
tests.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D113133