[clang][deps] Use MemoryBuffer in minimizing FS

This patch avoids unnecessarily copying contents of `mmap`-ed files into `CachedFileSystemEntry` by storing `MemoryBuffer` instead. The change leads to ~50% reduction of peak memory footprint when scanning LLVM+Clang via `clang-scan-deps`.

Depends on D115331.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D115043