Be a little more permissive in DynamicLoaderMacOS::CanLoadImage

If we can't find the "is dyld locked" symbol, assume it is safe
to load the image unless we only have 1 image loaded - in which case
we are in _dyld_start and it is definitely NOT safe.

Also add a little better errors to that function, and better logging
in SBProcess.cpp.

<rdar://problem/30174817>

llvm-svn: 302327