When loading mach-o corefile, new fallback for finding images

When lldb is reading a user process corefile, it starts by finding
dyld, then finding the dyld_all_image_infos structure in dyld by
symbol name, then getting the list of loaded binaries.  If it fails
to find the structure by name, it can't load binaries.  There is
an additional fallback that this patch adds, which is to look for
this object by the section name it is stored in, if the symbol name
lookup fails.

Differential Revision: https://reviews.llvm.org/D140066
rdar://103369931