Use File.is_directory for C++ tree artifacts

[dzbarsky]

Replace all five _cc_internal.is_tree_artifact calls in C++ compilation with the native Starlark File.is_directory property. Open-source Bazel creates directory artifacts for generated source and header trees, and existing C++ module-map code already uses the same property.

Add configured-target regression tests covering generated source directories and generated header directories.

Validation: buildifier -mode=check and five passing C++ library analysis tests, including both generated-directory regression cases.

[trybka]

@pzembrod would like you to take a look at this one given the TreeArtifact overlap. Specifically is_tree_artifact has an open TODO to consider if it can be replaced with artifact.is_directory, which leads me to wonder if you had thoughts as to why it may not be trivial.