CachePruning: correct comment about file order. NFC

Summary: Actually the list of cached files is sorted by file size, not by last accessed time. Also remove unused file access time param for a helper function.

Reviewers: joker-eph, chandlerc, davide

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21639

llvm-svn: 273852