FR: Expose --per_file_copt to Starlark

[brentleyjones]

Description of the problem / feature request:

I would like to get the effective value of --per_file_copt in a Starlark aspect.

Feature requests: what underlying problem are you trying to solve with this feature?

In a Starlark aspect I'm collecting all of the copts that are used to compile Objective-C files. Currently I can get most of them through cc_common.get_memory_inefficent_command_line (is there a better way?), but that doesn't take into account --per_file_copt.

It seems a fully Starlark objc_library would need this as well, as it's currently relying on some native code to allow this flag to influence the compile action.

What's the output of bazel info release?

release 5.0.0