[clang][modules] Deserialize submodules lazily (#194968)
This PR implements on-demand deserialization of `Module` objects from
PCM files. This is motivated by dependency scanning, where eager
deserialization of submodules turns out to be very expensive and
typically unnecessary. The core of this patch is the introduction of
`ModuleRef`, which may either be a pointer to `Module`, a pointer to new
`ExternalSubmoduleSource` (implemented by `ASTReader`) and
`serialization::SubmoduleID`, both, or none (null). Dereferencing
`ModuleRef` ensures the `Module` is deserialized if possible.
This replaces `ASTReader::UnresolvedModuleRefs` and changes the
structure of a PCM file a bit, most importantly introducing new
`SUBMODULE_CHILD` record that enables hooking up the laziness into the
qualified by-name lookup that uses `Module::SubModuleIndex`.
This speeds up dependency scanning by ~5.5%.
