[clang][modules] Deserialize included files lazily

In D114095, `HeaderFileInfo::NumIncludes` was moved into `Preprocessor`. This still makes sense, because we want to track this on the granularity of submodules (D112915, D114173), but the way this information is serialized is not ideal. In `ASTWriter`, the set of included files gets deserialized eagerly, issuing lots of calls to `FileManager::getFile()` for input files the PCM consumer might not be interested in.

This patch makes the information part of the header file info table, taking advantage of its lazy deserialization which typically happens when a file is about to be included.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D155131