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