Remove or change <filesystem> usage in project

[phated]

As noted in WebAssembly/wasi-sdk#125, the <filesystem> library cannot be used in wasm/wasi. This project seems stuck on wasi-sdk 12 because it contained a bug that didn't disable loading that filesystem header, but if that code path is every taken, it'd crash.

@adr1anh noted this in an internal issue, and had fixes to use mkdir instead; this seemed to have been blocked as "not portable".

We should either #ifndef around all code using <filesystem> or switch to mkdir. It seems this can be made portable by using _mkdir on Windows.

[phated]

I believe this is the LLVM patch that did it: https://reviews.llvm.org/D94921