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.