[Support][CachePruning] Fix regression in pruning interval

Fixed broken comparison.
borked by: rL284966 (see: https://reviews.llvm.org/D25730).

Differential Revision: https://reviews.llvm.org/D40119

This is a second attempt to commit this.
The first attempt broke lld and gold tests that had been written against
the incorrect behaivour.

llvm-svn: 318524