SourceManager: Clarify that FileInfo always has a ContentCache, NFC

It turns out that `FileInfo` *always* has a ContentCache. Clarify that
in the code:
- Update the private version of `SourceManager::createFileID` to take a
  `ContentCache&` instead of `ContentCache*`, and rename it to
  `createFileIDImpl` for clarity.
- Change `FileInfo::getContentCache` to return a reference.

Differential Revision: https://reviews.llvm.org/D89554