This PR adds new `ModuleCache` interface to Clang's implicitly-built
modules machinery. The main motivation for this change is to create a
second implementation that uses a more efficient kind of
`llvm::AdvisoryLock` during dependency scanning.
In addition to the lock abstraction, the `ModuleCache` interface also
manages the existing `InMemoryModuleCache` instance. I found that
compared to keeping these separate/independent, the code is a bit
simpler now, since these are two tightly coupled concepts. I can
envision a more efficient implementation of the `InMemoryModuleCache`
for the single-process case too, which will be much easier to implement
with the current setup.
This is not intended to be a functional change.
c84d8e8f1c