[ThinLTO/gold] Perform cache pruning when cache directory specified

Summary:
As pointed out in the review for D37993, for consistency with other
linkers, gold plugin should perform cache pruning whenever there is a
cache directory specified, which will use the default cache policy.

Reviewers: pcc

Subscribers: llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D43389

llvm-svn: 325830