Introduce the swift_common.get_toolchain helper function

This is change 1 of _N_ to migrate the Swift build rules to use new-style Bazel toolchains. This change:

*   Updates the toolchain configuration rules to wrap the `SwiftToolchainInfo` provider in `platform_common.ToolchainInfo`, for future use by `toolchain()` rules.
*   Funnels all toolchain access through a new `swift_toolchain.get_toolchain()` function that looks up the toolchain using `ctx.toolchains`, falling back to the implicit attribute.

Since `ctx.toolchains` isn't defined yet, the second change is a no-op but it simplifies future parts of the migration.

PiperOrigin-RevId: 439638938
(cherry picked from commit 73b248d)
Signed-off-by: Brentley Jones <github@brentleyjones.com>

apple/internal/apple_framework_import.bzl

@@ -33,7 +33,6 @@ load(
 load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain", "use_cpp_toolchain")
 load(
     "@build_bazel_rules_swift//swift:swift.bzl",
-    "SwiftToolchainInfo",
     "swift_clang_module_aspect",
     "swift_common",
 )
@@ -255,7 +254,7 @@ def _apple_dynamic_framework_import_impl(ctx):
 
     if "apple._import_framework_via_swiftinterface" in features and framework.swift_interface_imports:
         # Create SwiftInfo provider
-        swift_toolchain = ctx.attr._swift_toolchain[SwiftToolchainInfo]
+        swift_toolchain = swift_common.get_toolchain(ctx, attr = "_swift_toolchain")
         swiftinterface_files = framework_import_support.get_swift_module_files_with_target_triplet(
             swift_module_files = framework.swift_interface_imports,
             target_triplet = target_triplet,
@@ -321,7 +320,7 @@ def _apple_static_framework_import_impl(ctx):
     additional_objc_providers = []
     additional_objc_provider_fields = {}
     if framework.swift_interface_imports or framework.swift_module_imports or has_swift:
-        toolchain = ctx.attr._swift_toolchain[SwiftToolchainInfo]
+        toolchain = swift_common.get_toolchain(ctx, attr = "_swift_toolchain")
         providers.append(SwiftUsageInfo())
 
         # The Swift toolchain propagates Swift-specific linker flags (e.g.,
@@ -390,7 +389,7 @@ def _apple_static_framework_import_impl(ctx):
 
     if "apple._import_framework_via_swiftinterface" in features and framework.swift_interface_imports:
         # Create SwiftInfo provider
-        swift_toolchain = ctx.attr._swift_toolchain[SwiftToolchainInfo]
+        swift_toolchain = swift_common.get_toolchain(ctx, attr = "_swift_toolchain")
         swiftinterface_files = framework_import_support.get_swift_module_files_with_target_triplet(
             swift_module_files = framework.swift_interface_imports,
             target_triplet = target_triplet,

apple/internal/apple_xcframework_import.bzl

@@ -18,7 +18,11 @@ load("@bazel_skylib//lib:dicts.bzl", "dicts")
 load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain", "use_cpp_toolchain")
 load("@build_bazel_apple_support//lib:apple_support.bzl", "apple_support")
-load("@build_bazel_rules_swift//swift:swift.bzl", "SwiftToolchainInfo", "swift_clang_module_aspect", "swift_common")
+load(
+    "@build_bazel_rules_swift//swift:swift.bzl",
+    "swift_clang_module_aspect",
+    "swift_common",
+)
 load("//apple:providers.bzl", "AppleFrameworkImportInfo")
 load(
     "//apple/internal:apple_toolchains.bzl",
@@ -557,7 +561,7 @@ def _apple_dynamic_xcframework_import_impl(ctx):
 
     if "apple._import_framework_via_swiftinterface" in features and xcframework_library.swift_module_interface:
         # Create SwiftInfo provider
-        swift_toolchain = ctx.attr._toolchain[SwiftToolchainInfo]
+        swift_toolchain = swift_common.get_toolchain(ctx, attr = "_swift_toolchain")
         providers.append(
             framework_import_support.swift_info_from_module_interface(
                 actions = actions,
@@ -634,7 +638,7 @@ def _apple_static_xcframework_import_impl(ctx):
     if xcframework.files_by_category.swift_interface_imports or \
        xcframework.files_by_category.swift_module_imports or \
        has_swift:
-        swift_toolchain = ctx.attr._toolchain[SwiftToolchainInfo]
+        swift_toolchain = swift_common.get_toolchain(ctx, attr = "_swift_toolchain")
         providers.append(SwiftUsageInfo())
 
         # The Swift toolchain propagates Swift-specific linker flags (e.g.,
@@ -688,7 +692,7 @@ def _apple_static_xcframework_import_impl(ctx):
 
     if "apple._import_framework_via_swiftinterface" in features and xcframework_library.swift_module_interface:
         # Create SwiftInfo provider
-        swift_toolchain = ctx.attr._toolchain[SwiftToolchainInfo]
+        swift_toolchain = swift_common.get_toolchain(ctx, attr = "_swift_toolchain")
         providers.append(
             framework_import_support.swift_info_from_module_interface(
                 actions = actions,