[flang] Merge pull request flang-compiler/f18#1047 from CarolineConcatto/llvm_ostream

[LLVMify F18] Replace the use std::ostream with LLVM streams llvm::ostream

Original-commit: flang-compiler/f18@a65b690ef00351bd6bc02593ddbd45fa240e9cbc
Reviewed-on: https://github.com/flang-compiler/f18/pull/1047