Add Metal Performance Shaders

[tbetcke]

Hi,

this is a draft pull request to add Metal Performance Shaders.

I am trying to add the MetalPerformanceShaders framework. I have followed some other PRs in how to add frameworks.
But when I am running cargo run --bin header-translator I am getting the error message

─ERROR unknown unexposed attribute, name="MPS_ENUM_AVAILABLE_STARTING", fnlike=true

How do I update the libraries to deal with this? My progress so far is visible in this pull request.

[madsmtm]

Thanks for beginning on this!

I am getting the error message [...]

You probably have to update something in header-translator/src/unexposed_attr.rs, close to the other availability macros.

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Additionally, for the header translator to work, you have to create a fork of https://github.com/madsmtm/icrate-generated, commit the generated changes to that, and update the Git submodule in this PR to point to the commit in your fork (no need to create a PR to the repo).

[madsmtm]

Nit: Missing value for tvos.

[madsmtm]

One troublesome thing about MPS is that it's actually an umbrella framework containing many smaller (private) frameworks. In 03d7a74, I added preliminary support for this, though I think more work is still needed for this to work fully.

See also #640.