[PATCH] D18422: Add Cache Pruning support

[Mehdi Amini via llvm-commits]

This is copy-pasted-adapted from module pruning. I plan to refactor module pruning to use this at some point.

> On Mar 24, 2016, at 9:51 AM, Bruno Cardoso Lopes <bruno.cardoso at gmail.com> wrote:
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> bruno added a subscriber: bruno.
> bruno added a comment.
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> It might be worth looking at how clang implements pruning for old modules. The option in question is -fmodules-prune-interval.
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> http://reviews.llvm.org/D18422
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