[ThinLTO][CachePruning] explicitly disable pruning

In https://reviews.llvm.org/rL321077 and https://reviews.llvm.org/D41231 I fixed a regression in the c-api which prevented the pruning from being *effectively* disabled.

However this approach, helpfully recommended by @labath, is cleaner.
It is also nice to remove the weasel words about effectively disabling from the api comments.

Differential Revision: https://reviews.llvm.org/D41497

llvm-svn: 321376