The hash map mechanism that was already in use for reading swiftmodule
files on macOS is now in use also on Linux. The output replacing
mechanism has been also reworked so that:
* frontend module emission modes have the remapping done directly in
the internal frontend options instead of painstakingly modifying input
flags (this requires a patch on the swift headers though)
* object emission mode is silenced to be just a type checking pass,
thus producing no output files
* all other passes but some debugging and version related ones become
noops
The open file read redirection uses a global weak pointer instance to
maximize robustness in the face of possibly multi-threaded calls to open
happening while `main` is exiting. Possibly overkill, but better safe
than sorry.
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.
This should cover `-merge-modules` mode.
Dumping of the configuration to the target files was moved to a
separate pair of header/source files, as now it is also done in
`SwiftOutputRewrite.cpp`.
