Fix a few bugs with Mach-O corefile loading, plus perf

In ProcessMachCore::LoadBinariesViaMetadata(), if we did
load some binaries via metadata in the core file, don't
then search for a userland dyld in the corefile / kernel
and throw away that binary list.  Also fix a little bug
with correctly recognizing corefiles using a `main bin spec`
LC_NOTE that explicitly declare that this is a userland
corefile.

LocateSymbolFileMacOSX.cpp's Symbols::DownloadObjectAndSymbolFile
clarify the comments on how the force_lookup and how the
dbgshell_command local both have the same effect.

In PlatformDarwinKernel::LoadPlatformBinaryAndSetup, don't
log a message unless we actually found a kernel fileset.

Reorganize ObjectFileMachO::LoadCoreFileImages so that it delegates
binary searching to DynamicLoader::LoadBinaryWithUUIDAndAddress and
doesn't duplicate those searches.  For searches that fail, we would
perform them multiple times in both methods.  When we have the
mach-o segment vmaddrs for a binary, don't let LoadBinaryWithUUIDAndAddress
load the binary first at its mach-o header address in the Target;
we'll load the segments at the correct addresses individually later
in this method.

DynamicLoaderDarwin::ImageInfo::PutToLog fix a LLDB_LOG logging
formatter.

In DynamicLoader::LoadBinaryWithUUIDAndAddress, instead of using
Target::GetOrCreateModule as a way to find a binary already registered
in lldb's global module cache (and implicitly add it to the Target
image list), use ModuleList::GetSharedModule() which only searches
the global module cache, don't add it to the Target.  We may not
want to add an unstripped binary to the Target.

Add a call to Symbols::DownloadObjectAndSymbolFile() even if
"force_symbol_search" isn't set -- this will turn into a
DebugSymbols call / Spotlight search on a macOS system, which
we want.

Only set the Module's LoadAddress if the caller asked us to do that.

Differential Revision: https://reviews.llvm.org/D150928
rdar://109186357