Use ghc::filesystem as fill on older GCC (e.g. RoboRIO).
This can be removed once all GCC platforms have upgraded to 8.1 or later.
File open functionality has been retained from LLVM but moved to "fs" namespace
and tweaked for improved consistency with std::filesystem (e.g. error_code is
passed by reference instead of returned).
Also update WPILibC's Filesystem functions to return std::string.
This makes code easier to read and more consistent between C++ and Java.
Also update clang-format settings to always add a line break (even if no braces are used).
This avoids a number of shutdown use-after-free races by controlling the
destruction order. It also is a prerequisite to making the internal
interfaces mockable for unit testing.
