[Flang] Use PrintModulePass to print LLVM IR from the frontend (#95142)

The Flang frontend currently prints LLVM IR modules using
llvm::Module::print(); this works for default cases, but skips some of
the logic that IR printer passes use, specifically the use of the
--write-experimental-debuginfo flag to control debug info format. This
patch replaces the use of print() with the PrintModulePass, bringing the
printing behaviour to parity with clang's frontend.