CachePruning: Allow limiting the number of files in the cache directory.

The default limit is 1000000 but it can be configured with a cache
policy. The motivation is that some filesystems (notably ext4) have
a limit on the number of files that can be contained in a directory
(separate from the inode limit).

Differential Revision: https://reviews.llvm.org/D40327

llvm-svn: 318857