libstdc++: Call regex_traits::transform_primary() only when necessary [PR98723]

libstdc++: Call regex_traits::transform_primary() only when necessary [PR98723]

This is both a performance optimization and a partial fix for PR 98723.

This commit fixes the issue for bracket expressions that do not depend
on the locale's collation facet. Examples:

 * Character ranges ([a-z]) when std::regex::collate is not set
 * Character classes ([:alnum:])
 * Individual characters ([abc])

Signed-off-by: Luca Bacci <luca.bacci982@gmail.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/98723
	* include/bits/regex_compiler.tcc (_BracketMatcher::_M_apply):
	Only use transform_primary when an equivalence set is used.