Make C++ toolchain optional

Currently, if you transition a go_binary, you must have a C++ toolchain
for it even if it's building in pure mode without cgo.

By making this toolchain optional, we push the error from a
toolchain-resolution-time error to an analysis-time error only if a
toolchain is both _needed_ and not found.

WORKSPACE

@@ -167,3 +167,12 @@ http_archive(
 load("@io_bazel_stardoc//:setup.bzl", "stardoc_repositories")
 
 stardoc_repositories()
+
+local_repository(
+    name = "apple_federation",
+    path = "/Volumes/Source/github.pie.apple.com/bazel/apple_federation",
+)
+
+load("@apple_federation//common:repositories.bzl", "common_deps")
+
+common_deps()

go/private/context.bzl

@@ -781,7 +781,13 @@ cgo_context_data = rule(
             default = "@bazel_tools//tools/osx:current_xcode_config",
         ),
     },
-    toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
+    toolchains = [
+        # In pure mode, a C++ toolchain isn't needed when transitioning.
+        # But if we declare a mandatory toolchain dependency here, a cross-compiling C++ toolchain is required at toolchain resolution time.
+        # So we make this toolchain dependency optional, so that it's only attempted to be looked up if it's actually needed.
+        # Optional toolchain support was added in bazel 6.0.0.
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False) if hasattr(config_common, "toolchain_type") else "@bazel_tools//tools/cpp:toolchain_type",
+    ],
     fragments = ["apple", "cpp"],
     provides = [CgoContextInfo],
     doc = """Collects information about the C/C++ toolchain. The C/C++ toolchain