Don't try to cache invocations with /Zi

We currently have no support for compiler invocations which generate external debug files (.pdb files). Technically, it would be possible
to support that - simply by copying those files into/out of the cache. For now, let's just not try to cache that. /Z7, which generates
debug information embedded into the binary, should work fine though.

Since I might eventually add support for /Zi, I decided not to introduce a cache statistic field for this.