Add a parameter to Symbols::DownloadObjectAndSymbolFile() to control

whether we try to call an external program to load symbols unconditionally,
or if we check the user's preferences before calling it.

ProcessMachCore now sets CanJIT to false - we can't execute code in a core file.

DynamicLoaderDarwinKernel::OSKextLoadedKextSummary::LoadImageUsingMemoryModule changed
to load the kernel from an on-disk file if at all possible.
Don't load the kext binaries out of memory from the remote systems - their linkedit doesn't
seem to be in a good state and we'll error out down in SymbolVendorMacOSX if we try to use
the in-memory images.
Call Symbols::DownloadObjectAndSymbolFile to get the kext/kernel binary -- the external
program may be able to give us a file path on the local filesystem instead of reading
the binary / dSYM over a network drive every time.  Fall back to calling 
Target::GetSharedModule() like before if DownloadObjectAndSymbolFile fails.

llvm-svn: 165471