[libstdc++/71500] make back reference work with icase

[Tim Shen via libstdc++]

This fixes the follow-up comments in 71500.

Back-reference matching is different from other matching, as the
content the back-reference refers to is at "run-time", aka during
regex_match(), not regex() compilation.

For compilation we do have an abstraction layer to catch all
comparison customizations, namely _M_translator in regex_compiler.h.
Until this patch, we don't have an abstraction for "run-time"
matching. I believe that back-reference is the only place that needs
run-time matching, so I just build a _Backref_matcher in
regex_executot.tcc.

Tested on x86_64-linux-gnu.

Thanks!

-- 
Regards,
Tim Shen
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