[clang][deps] Squash caches for original and minimized files

The minimizing and caching filesystem used by the dependency scanner keeps minimized and original files in separate caches.

This setup is not well suited for dealing with files that are sometimes minimized and sometimes not. Such files are being stat-ed and read twice, which is wasteful and also means the two versions of the file can get "out of sync".

This patch squashes the two caches together. When a file is stat-ed or read, its original contents are populated. If a file needs to be minimized, we give the minimizer the already loaded contents instead of reading the file again.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D115346