[ParseableInterface] Add ‘forwarding modules’

A ‘forwarding module’ is a YAML file that’s meant to stand in for a .swiftmodule file and provide an up-to-date description of its dependencies, always using modification times.

When a ‘prebuilt module’ is first loaded, we verify that it’s up-to-date by hashing all of its dependencies. Since this is orders of magnitude slower than reading mtimes, we’ll install a `forwarding module` containing the mtimes of the now-validated dependencies.

test/ParseableInterface/ModuleCache/Inputs/check-is-forwarding-module.py

@@ -0,0 +1,7 @@
+import sys
+
+with open(sys.argv[1], 'r') as yaml_file:
+    # Forwarding files are YAML files that start with '---'
+    if yaml_file.read(3) != '---':
+        print("swiftmodule '%s' is not a forwarding module!")
+        sys.exit(1)