Rework inclusion handling about rules_ll

[neqochan]

(transitive_)(relative_)(angled_)includes now handle -iquote and -I. We have absolute and workspace-relative includes working, but cross-workspace includes are missing. This is particularly relevant for Projects building on top of LLVM.

We need to be able to reference LLVM and Clang headers from outside the @llvm-project workspace. Since the current implementation uses rules_cc for these targets we need some sort of compatibility layer. Either rules_ll must understand cc_library targets better, or we add ll_* targets to the existing cc_* overlay.

We did not encounter a need for -idirafter, -isystem and -isystem-after, so this is postponed.

[aaronmondal]

After adding module support we get some unwanted warnings in precompilation steps regarding unused -iquote.... I'm also considering whether we want to change the APIs to have finer-grained control regarding transitivity. Something like

attr
public_attr
transitive_attr

where public_attr is only made available to direct dependencies and transitive_attr is available to all downstream dependencies in the build graph. We need to be careful not to bloat the user interface for ll_library though 😅

[aaronmondal]

We now removed all transitive_* attributes in favor of exposed_* attributes which don't carry dependencies transitively. I think this is good for now. We can create another issue if this needs another rework.