[committed 0/3] libstdc++: Refactor filesystem::path string conversions

[Jonathan Wakely]

This is a series of three patches to simplify the logic used to
construct a std::filesystem::path from strings/ranges of arbitrary
characters, and fix one bug.

The simpler logic also avoids some unnecessary string constructions
when a string view could be used instead. Previously a string view was
only used for strings and pairs of pointers. Now contiguous iterators
will be detected (using the concept in C++20, or by handling the
__normal_iterator wrapper in C++17).

The bug fix should be backported, the rest would be safe but doesn't
need to be.