I got a new (M2) macbook and did a system migration from my old machine. I hoped the problem would be gone, but it wasn’t.
I have since installed Xcode and did the following horrible hack, which works. After installing pymc in my conda environment, I have two versions of clang. The one in the environment and the Xcode version in /usr/bin. I wondered whether the Xcode version might be fine, so I went into the bin dir in the conda environment and replaced clang and clang++ with symbolic links to the versions in /usr/bin. I’m not going to put the commands here, because I don’t want anyone else to just paste my commands into their shell without knowing what they’re doing. If you do know what you’re doing, you will know how to do what I just described 
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Summary: using the xcode version of clang works fine. It’s also a horrible hack and is likely to cause an issue at some later point and need to be undone. For now though I can at least use pymc again. I have no idea why the conda installed version of clang cannot find the standard headers. All web pages I read about this give solutions for Ubuntu, not OS X.