[lldb] parallelize calling of Module::PreloadSymbols()

If LLDB index cache is enabled and everything is cached, then loading of debug
info is essentially single-threaded, because it's done from PreloadSymbols()
called from GetOrCreateModule(), which is called from a loop calling
LoadModuleAtAddress() in DynamicLoaderPOSIXDYLD. Parallelizing the entire
loop could be unsafe because of GetOrCreateModule() operating on a module
list, so instead move only the PreloadSymbols() call to Target::ModulesDidLoad()
and parallelize there, which should be safe.

This may greatly reduce the load time if the debugged program uses a large
number of binaries (as opposed to monolithic programs where this presumably
doesn't make a difference). In my specific case of LibreOffice Calc this reduces
startup time from 6s to 2s.

Differential Revision: https://reviews.llvm.org/D122975