[toolchains] Add helper for extracting action names from cc_action_type rules

[armandomontanez]

Many rules use cc_common.get_tool_for_action() to create custom rules that employ binaries from the current C/C++ toolchain. The status-quo today is to use the string constants exposed in Starlark files. Since these are now represented as rules, it's plausible to express the action types by referencing the source-of-truth cc_action_type rule. Today this is not possible because the providers for toolchain rules are considered private and not exposed to prevent projects from relying on implementation details of the toolchain rules.

Proposed solution

The simplest path forward is to add a action_name_from_action_type(target) helper that takes a target and extracts the action name from the action type. This will work as a bridge so user can do things like:

def _foo_rule_impl(ctx):
    # ...
    tool_path = cc_common.get_tool_for_action(
        feature_configuration = action_name_from_action_type(ctx.attr._objcopy),
        action_name = action_name,
    )
    # Run tool...


foo_rule = rule(
    implementation = _foo_rule_impl,
    attrs = {
        "_objcopy": attr.label(
            default = "@rules_cc//cc/toolchains/actions:objcopy_embed_data",
        ),
    },
    toolchains = use_cpp_toolchain(),
    fragments = ["cpp"],
)

[pzembrod]

I must admit that I don't yet fully grasp your proposal (I'm still ramping up on the C++ rules), but I'm happy to discuss this in more detail.