This patch modifies ParseableInterfaceBuilder::CollectDepsForSerialization to
avoid serializing dependencies from the runtime resource path into the
swiftmodules generated from .swiftinterface files. This means the module cache
should now be relocatable across machines.
It also modifies ParseableInterfaceModuleLoader to never add any dependencies
from the module cache and prebuilt cache to the dependency tracker (in addition
to the existing behaviour of not serializing them in the generated
swiftmodules). As a result, CollectDepsForSerialization no longer checks if the
dependencies it is given come from the cache as they are provided by the
dependency tracker. It now asserts that's the case instead.
Updates the subinvocation that builds the parseable interface to respect the
-track-system-dependencies flag of the top-level invocation if present, by
including system dependencies in the produced .swiftmodule.
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.
