`allowlist_absolute_include_directories` on `cc_args` is unusable on Windows

[stevenlr]

Hello, I'm trying to write a rules-based toolchain for a host (non-hermetic) Clang on Windows and have run into an issue to replicate the former cxx_builtin_include_directories field from cc_common.create_cc_toolchain_config_info.

The code to check that the paths given to allowlist_absolute_include_directories are absolute only checks that the paths starts with /, which is not compatible with Windows paths. Because of this there is no other way that I know of to make header files inclusion happy.

[keith]

cc @ma-oli

[stevenlr]

Fixed by #477.