Eliminates a dependency on LLVM with regards to the set of MinGW .def files shipped with Zig. This implementation is largely based on the LLVM implementation (specifically COFFModuleDefinition.cpp and COFFImportFile.cpp).

How this was tested

I developed/tested this in a separate repository here (the def.zig/implib.zig files are 1:1 between that repository and this PR)

I have a branch of zig here that adds a zig implib command that takes in a .def file and outputs two files: a .pp.def file that's the result of preprocessing with Aro (this is something that always happens during buildImportLib), and a .lib file that's the result of calling llvm_bindings.WriteImportLibrary after preprocessing.

I then have a gen.zig script here that walks Zig's lib/libc/mingw directory and spawns zig implib to create .pp.def/.lib pairs for every .def and .def.in file found.

Finally, I have a check.zig script here that walks a directory of these generated pairs and confirms that the new implementation generates a byte-for-byte identical .lib file for each .pp.def input.

The check.zig script passes cleanly for the entire lib/libc/mingw directory when targeting all supported windows-gnu targets: x86_64, x86, thumb, and aarch64 (see here for why these are specified).

(note that the byte-for-byte nature means that the implementation has inherited a few quirks from LLVM around size counts/padding; those quirks have been kept for now just to ensure there's no possible breakage, but that should be revisited when eliminating the LLD dependency for COFF)

How this fits into the bigger picture

There are effectively two parts to this implementation:

• .def file parsing
• .lib file writing (import library using the COFF archive file format)

In theory, the second part could be made redundant once the self-hosted COFF linker is ready, since actually writing out the archive file format is not a necessary component of linking--all the required information exists in the .def file.

However, the archive file writing implementation is not (in principle) COFF-specific (although the current implementation is), so in the future:

• the self-hosted COFF linker could just take the output of .def file parsing as an input and skip the COFF import library generation
• the import library writing code could be repurposed (where possible) for zig ar: a drop-in llvm-ar replacement #9828

If #17807 is specifically about MinGW .def files, then this closes #17807