libstdc++: Remove <memory_resource> dependency from <regex> [PR 92546]

Unlike the other headers that declare alias templates in namespace pmr,
<regex> includes <memory_resource>. That was done because the
pmr::string::const_iterator typedef requires pmr::string to be complete,
which requires pmr::polymorphic_allocator<char> to be complete.

By using __normal_iterator<const char*, pmr::string> instead of the
const_iterator typedef we can avoid the completeness requirement.

This makes <regex> smaller, by not requiring <memory_resource> and its
<shared_mutex> dependency, which depends on <chrono>.  Backporting this
will also help with PR 97876, where <stop_token> ends up being needed by
<regex> via <memory_resource>.

libstdc++-v3/ChangeLog:

	PR libstdc++/92546
	* include/std/regex (pmr::smatch, pmr::wsmatch): Declare using
	underlying __normal_iterator type, not nested typedef
	basic_string::const_iterator.

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