Platforms aren't propagated in transitions

We have a mixed language build and spent a bit of time trying to understand why platforms aren't propagated (well, ignored) when building a `ios_application` target, but worked fine when building a `swift_library` one (the core library which the app depends on). When building the `ios_application` target, `--platforms` are ignored and the build fails with symbols from mixed platforms (some built with the host target and the final linking phase with the right one).

After a bit of investigating, we realized that adding the `//:command_line_options:platforms` to the transition outputs and setting it to the platforms declared in [@build_bazel_apple_support//platforms](https://github.com/bazelbuild/apple_support/blob/master/platforms/BUILD) fixes our issue.

When using only `--platforms` with the transition `platform_mappings` API, this works exactly as intended. A bit more logic change was involved to fix `apple_xcframework` when using `--platforms` which we should probably send a patch for separately.

I think the patch is rather straightforward, but I have a bunch of questions before I open a PR to discuss it further:

- It feels like the `master` branch is seeing a lot of work towards supporting exactly this, and maybe works only on Bazel 6.x? Would you still accept such contribution on the `bazel/5.x` branch?
- Do you know if it is expected behaviour that Bazel completely ignores any invalid values of `--platforms=` if a target doesn't make use of them/apply transitions based on this flag. This sounds like a bug to me.

```patch
apple/internal/transition_support.bzl | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/apple/internal/transition_support.bzl b/apple/internal/transition_support.bzl
index 4f841f76..f651e034 100644
--- a/apple/internal/transition_support.bzl
+++ b/apple/internal/transition_support.bzl
@@ -126,16 +126,23 @@ def _command_line_options(
A dictionary of `"//command_line_option"`s defined for the current target.
"""

+ # Derive platform label defined in @build_bazel_apple_support//platforms.
+ #
+ # Doing this transition allows to set number of constrains defined in @build_bazel_apple_support
+ # used by other rules (e.g. rules_rust).
+ cpu_string = _cpu_string(
+ cpu = cpu,
+ platform_type = platform_type,
+ settings = settings,
+ )
+
output_dictionary = {
"//command_line_option:apple configuration distinguisher": "applebin_" + platform_type,
"//command_line_option:apple_platform_type": platform_type,
"//command_line_option:apple_split_cpu": cpu if cpu else "",
"//command_line_option:compiler": settings["//command_line_option:apple_compiler"],
- "//command_line_option:cpu": _cpu_string(
- cpu = cpu,
- platform_type = platform_type,
- settings = settings,
- ),
+ "//command_line_option:cpu": cpu_string,
+ "//command_line_option:platforms": "@build_bazel_apple_support//platforms:{}".format(cpu_string),
"//command_line_option:crosstool_top": (
settings["//command_line_option:apple_crosstool_top"]
),
@@ -300,6 +307,7 @@ _apple_rule_base_transition_outputs = [
"//command_line_option:macos_minimum_os",
"//command_line_option:tvos_minimum_os",
"//command_line_option:watchos_minimum_os",
+ "//command_line_option:platforms",
]
_apple_universal_binary_rule_transition_outputs = _apple_rule_base_transition_outputs + [
"//command_line_option:ios_multi_cpus",
@@ -415,7 +423,6 @@ _xcframework_transition = transition(
inputs = _apple_rule_common_transition_inputs,
outputs = _apple_rule_base_transition_outputs + [
"@build_bazel_rules_swift//swift:emit_swiftinterface",
- "//command_line_option:platforms",
],
)

--
2.32.1 (Apple Git-133)
```

cc @boxdot

Start in apple/internal/transition_support.bzl, especially _command_line_options and the transition output lists. Reproduce an ios_application build using --platforms alongside a swift_library in a mixed-language build, then verify that linking no longer combines host and target platforms. Resolve the Bazel 5.x compatibility and invalid --platforms questions before considering the issue done.

Written by the indexing model from the issue text.