Fixes #4995.

Unblocks libcxx test std/re/re.regex/re.regex.construct/bad_range.pass.cpp.

Note that I did not add test coverage for regular expressions like [[.a.]-e]: After this change, this regex is rejected as well (rather than parsed incorrectly). However, such regular expressions should actually be accepted (!), because [.xxx.] is not a set of characters, but (kind of) a single character if xxx is the name of a valid collating element. But support of collating symbols is currently so terribly broken that it is basically guaranteed to produce wrong results (see #994 (comment)), so I think it would be a major disservice to users if we actually accepted this regex without a major overhaul of collation support in the implementation.