[Support][CachePruning] Disable cache pruning regression fix

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

Previously, Interval was unsigned (see: CachePruning.h), replacing the type with std::chrono::seconds (which is signed) causes a regression in behaviour because the c-api intends negative values to translate to large positive intervals to *effectively* disable the pruning (see comments on: setCachePruningInterval()).

Differential Revision: https://reviews.llvm.org/D41231

llvm-svn: 321077