Make ConvertStandardToLLVMPass extendable with other patterns

Extend the LLVM lowering pass to accept callbacks that construct an instance of
(a subclass of) LLVMTypeConverter and populate a list of conversion patterns.
These callbacks will be called when the pass processes a module and their
results will be used to set up the dialect conversion infrastructure.  Clients
can now provide additional conversion patterns to avoid the need of
materializing type conversions between LLVM and other types.

PiperOrigin-RevId: 256532415