[Tests][SwiftPM] Also include _get_toolchain_path in swiftpm.py.

We need the `_get_toolchain_path` function in `swiftpm.py` also.

Author: al45tair

utils/swift_build_support/swift_build_support/products/swiftpm.py

@@ -182,3 +182,16 @@ def get_dependencies(cls):
                 xctest.XCTest,
                 llbuild.LLBuild,
                 swift_testing.SwiftTesting]
+
+def _get_toolchain_path(host_target, product, args):
+    # TODO check if we should prefer using product.install_toolchain_path
+    # this logic initially was inside run_build_script_helper
+    # and was factored out so it can be used in testing as well
+
+    toolchain_path = product.host_install_destdir(host_target)
+    if platform.system() == 'Darwin':
+        # The prefix is an absolute path, so concatenate without os.path.
+        toolchain_path += \
+            targets.darwin_toolchain_prefix(args.install_prefix)
+
+    return toolchain_path