Remove main repo dependencies on LLVM

[creachadair]

Right now the main Kythe repo depends on LLVM in ways other than just the C++ indexer itself. It's fine for the C++ indexer to depend on LLVM, but the rest of the main repository shouldn't.

As far as I know the only reason we depend on it is for some library components, and we may be able to get those from Abseil instead.

[creachadair]

I believe this may be almost done:

$ bazel query --output=label 'rdeps(//..., @org_llvm//..., 1) intersect //...' | sort
//kythe/cxx/extractor:cmdlib
//kythe/cxx/extractor:command_line_utils
//kythe/cxx/extractor:cxx_details
//kythe/cxx/extractor:index_pack
//kythe/cxx/extractor:lib
//kythe/cxx/extractor:objc_bazel_support_library
//kythe/cxx/extractor:path_utils
//kythe/cxx/extractor:supported_language
//kythe/cxx/extractor:testlib
//kythe/cxx/indexer/cxx:clang_utils
//kythe/cxx/indexer/cxx:google_flags_library_support
//kythe/cxx/indexer/cxx:graph_observer
//kythe/cxx/indexer/cxx:imputed_constructor_library_support
//kythe/cxx/indexer/cxx:indexed_parent_iterator
//kythe/cxx/indexer/cxx:indexed_parent_map
//kythe/cxx/indexer/cxx:indexer_ast_hooks
//kythe/cxx/indexer/cxx:indexer_library_support
//kythe/cxx/indexer/cxx:indexer_pp_callbacks
//kythe/cxx/indexer/cxx:kythe_graph_observer
//kythe/cxx/indexer/cxx:lib
//kythe/cxx/indexer/cxx:marked_source
//kythe/cxx/indexer/cxx:proto_library_support
//kythe/cxx/indexer/cxx:semantic_hash
//kythe/cxx/indexer/cxx:testlib
//kythe/cxx/indexer/cxx:type_map
//kythe/cxx/indexer/cxx:vfs
//kythe/cxx/tools/fyi:fyilib
//third_party/llvm/src:clang_builtin_headers
//third_party/llvm/src:preprocessor_util_lib

Note that (almost?) all the direct LLVM dependencies are now the C++ indexer or extractor (which necessarily requires LLVM), or LLVM third_party components.

Is there anything further we need to address here?

[creachadair]

As far as I can tell, this is done. Feel free to reopen it if I missed something.