Auto merge of #12498 - upsuper:msvc-bindgen, r=emilio

Support generating bindings for MSVC

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This pull request includes several patches to the binding generating script to make it work with MSVC. The generated files are not updated because they may not be compatible with other platforms which the majority of developers are working on.

Only `regen.py` is modified. The two `.sh` files are not. Those files are very platform-specific and I don't think it's worth to make them work on Windows at all, and my hope is that we can get rid of those files and only use `regen.py`. I imagine the only left steps to get there are:

1. make clang version detectable via `bindgen` so that we get reliable clang version without needing to duplicate the library searching work (which has already been done in `clang-sys`)
2. checkout the git repo inside the python script and run `cargo build` there

BTW, it seems to me nightly Rust is not required to build `bindgen`, and thus we can probably get rid of the `multirust` detection. (Even if we need that, I think we should prefer `rustup` and optionally fallback to `multirust`. I know `rustup` is not yet available on Homebrew, though...)

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it is a change to binding generating script

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r? @emilio

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