2.x cherry-pick: Add clang_deps attr to mixed_language_library (#1540)

[jschear]

At Square, we're using a headermap for the Swift generated header, so that the objective-C half of a module is able to import it using #import <Module/Module-Swift.h>.

This headermap target can't be placed in deps, as doing so creates a dependency cycle.

Here is an example of what the use case looks like: master...jschear:rules_swift:js/demo_swift_header_imports_in_clang_half

header_map(
    name = "swift_headermap",
    module_name = "MixedAnswer",
    propagate_include = False,
    # This relies on us knowing the naming scheme used for internal targets in mixed_language_library, which is not ideal.
    deps = ["MixedAnswer_swift"],
)

mixed_language_library(
    name = "MixedAnswer",
    hdrs = ["MixedAnswer.h"],
    clang_copts = [
        "-I$(execpath :swift_headermap)",
        "-I.",
    ],
    clang_deps = [":swift_headermap"],
    clang_srcs = [
        "MixedAnswer.m",
        "MixedAnswerPrivate.m",
        "MixedAnswerPrivate.h",
    ],
    enable_modules = True,
    module_name = "MixedAnswer",
    swift_srcs = [
        "MixedAnswer.swift",
    ],
    target_compatible_with = ["@platforms//os:macos"],
)