HANG: drcachesim config reader can infinite loop on some poorly-formed cache configurations

drcachesim/reader/config_reader.cpp includes loop-detection code to make sure the cache hierarchy is a DAG.

This loop detection code can (ironically) get stuck in an infinite loop if either (a) a cache refers to a non-existent cache
as its parent, or (b) there is a loop on the path from a cache towards memory that does _not_ include the first cache checked.

None of the config_reader tests include configurations with these mistakes.