use libcxx-devel

[h-vetinari]

This goes together with conda-forge/libcxx-feedstock#183 (and backports; all for conda-forge/clangdev-feedstock#310), though the builds from that have been marked broken for now, because without this PR, our existing C++ compiler setup would break (as current clangxx_{{ target }} only pulls in libcxx which doesn't contain the headers anymore after the split).

For now, this PR is only for review (the changes are at least being tested for llvm_rc builds) and to discuss how to pull this off without too much breakage. Approach for now is to introduce libcxx-devel as a wrapper around the existing libcxx that keeps all of its content (for now), and once this PR is merged, we can actually enforce the split in libcxx properly.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[h-vetinari]

After discussion in conda-forge/clangdev-feedstock#310, this leaves the existing run-export

clang-compiler-activation-feedstock/recipe/meta.yaml

Lines 113 to 115 in 52e3513

	run_exports:
	strong:
	- libcxx >={{ libcxx_major }}

untouched. We'll add the static libs back to libcxx itself so that cross-compilation keeps working.

If you want me to leave a comment about potential improvements under conda/ceps#77, please let me know @isuruf. Otherwise this should be ready :)

[h-vetinari]

Gentle ping @isuruf

[isuruf]

This is not needed since clangxx now depend on libcxx-devel

[h-vetinari]

Fair point. I guess we could just remove those dependencies outright here, as they're coming in through clangxx now.

[h-vetinari]

OTOH, it still serves as a version anchor (so we shouldn't just delete it). OK, let's try leaving things as-is then.