libstdc++: Fix constexpr functions to conform to older standards

Some constexpr functions were inadvertently relying on relaxed constexpr
rules from later standards.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (duration_cast): Do not use braces
	around statements for C++11 constexpr rules.
	* include/bits/stl_algobase.h (__lg): Rewrite as a single
	statement for C++11 constexpr rules.
	* include/experimental/bits/fs_path.h (path::string): Use
	_GLIBCXX17_CONSTEXPR not _GLIBCXX_CONSTEXPR for 'if constexpr'.
	* include/std/charconv (__to_chars_8): Initialize variable for
	C++17 constexpr rules.