This is useful when an upper limit on the cache size needs to be
controlled independently of the amount of the amount of free space.

One use case is a machine with a large number of cache directories
(e.g. a buildbot slave hosting a large number of independent build
jobs). By imposing an upper size limit on each cache directory,
users can more easily estimate the server's capacity.

Differential Revision: https://reviews.llvm.org/D34547

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306126 91177308-0d34-0410-b5e6-96231b3b80d8