[LLJIT] Generalize LLJITBuilder::ProcessSymbolsJITDylibSetupFunction.

For many interesting process-symbols setups we need access to the LLJIT instance
(e.g. to mangle symbols, or inspect the process triple). This patch updates the
ProcessSymbolsJITDylibSetupFunction to take an LLJIT reference and return the
process symbols JITDylib, which the callback must now create.