Move clang dependency to conda develop packages.

[bdice]

Description

This moves the clang package to the conda section instead of conda_and_requirements, since it is not pip-installable (the clang package on PyPI is not the same thing).

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

@gpucibot merge