Previous to this patch the code contained a workaround for the standard
defect
https://cplusplus.github.io/LWG/issue3657
where `std::filesystem::path` did not have a `std::hash` implementation.
This patch allows compiling against versions of the STL that contain the
fix to the above issue. This is done by running the compiler against
code defining `std::hash<std::filesystem::path>`: if compilation
succeeds, it means the fix is not there and we need to use the
workaround, contained in `PathHash.h.workaround`. Otherwise, the fix is
there and we use `PathHash.h.fixed` instead, which only includes the
standard headers included by `PathHash.h.workaround`, so that one is a
drop-in replacement of the other.
This replaces usages of `llvm::fs` and string manipulation with
`std::filesystem`, also replacing `std::string` with
`std::filesystem::path` where it made sense.
Moreover MD5 hashing used in macOS file remapping was replaced by
SHA256 hashing using a small header-only SHA256 C++ library with an
MIT license, https://github.com/okdshin/PicoSHA2.
File contents hashing was relocated to the newly created `file` library
for later planned reuse.
