[clang][deps] NFC: Remove redundant CompilerInstance reference

The `ModuleDepCollectorPP` class holds a reference to `ModuleDepCollector` as well as `ModuleDepCollector`'s `CompilerInstance`. The fact that these refer to the same object is non-obvious.

This patch removes the `CompilerInvocation` reference from `ModuleDepCollectorPP` and accesses it through `ModuleDepCollector` instead.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D111724