[Debuginfod] Prune cache after fetch.

Previously, the size of the debuginfod cache would grow without bound.
This change prunes the cache after a successful debuginfod lookup, as is
done in libdebuginfod.

The cache pruning behavior is configured by a new
DEBUGINFOD_CACHE_POLICY environment variable. The semantics of this are
the same as --thinlto_cache_policy.

Reviewed By: gulfem

Differential Revision: https://reviews.llvm.org/D145618