When ProcessMachCore has metadata for a binary, don't scan

Mach-O corefiles have several possible types of metadata for the
binaries that were running when the corefile was written.
ProcessMachCore will try to find these binaries, and load them.
When we have a hint, but could not find the binary, this change
makes ProcessMachCore not fall back to scanning the corefile looking
for ANY binary that it could load.  We sometimes have multiple binaries
present in the memory in a corefile, but only the correct binary
should be loaded, the others are data.

Differential Revision: https://reviews.llvm.org/D157168
rdar://112602508