Add a ThinLTO cache policy for controlling the maximum cache size in bytes.

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

llvm-svn: 306126