Add macOS CC toolchain

Author: gferon

swift/internal/swift_autoconfiguration.bzl

@@ -389,16 +389,25 @@ def _create_xcode_toolchain(repository_ctx):
         SWIFT_FEATURE_MODULE_MAP_NO_PRIVATE_HEADERS,
     ]
 
+    path_to_swiftc = repository_ctx.which("swiftc")
+    toolchain_root = path_to_swiftc.dirname
+
     repository_ctx.file(
         "BUILD",
         """
+load("@bazel_tools//tools/cpp:unix_cc_toolchain_config.bzl", "cc_toolchain_config")
 load(
     "@build_bazel_rules_swift//swift/internal:xcode_swift_toolchain.bzl",
     "xcode_swift_toolchain",
 )
 
 package(default_visibility = ["//visibility:public"])
 
+alias(
+    name = "current_swift_cc_toolchain",
+    actual = "@local_config_apple_cc//:toolchain"
+)
+
 xcode_swift_toolchain(
     name = "toolchain",
     features = [{feature_list}],
@@ -408,6 +417,7 @@ xcode_swift_toolchain(
                 '"{}"'.format(feature)
                 for feature in feature_values
             ]),
+            toolchain_root = toolchain_root,
         ),
     )
 

swift/internal/swift_toolchain.bzl

@@ -432,7 +432,7 @@ configuration options that are applied to targets on a per-package basis.
                 allow_single_file = True,
             ),
             "_cc_toolchain": attr.label(
-                default = Label("@build_bazel_rules_swift_local_config//:current_swift_cc_toolchain"),
+                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
                 doc = """\
 The C++ toolchain from which other tools needed by the Swift toolchain (such as
 `clang` and `ar`) will be retrieved.