[libc++] Wrong match results for POSIX basic regular expression `[]-_]`

[muellerj2]

The basic regular expression []-_] should match all characters in the character range ]-_ (i.e., ], ^ and _). But libc++ erroneously does not treat ]-_ as a character range and instead matches ], - and _.

Godbolt: https://godbolt.org/z/4WE8PdGPE