[flang][openacc] Only issue a warning when acc routine func is not found (#70964)

Do not issue a hard error when the function in acc routine directive is
not present in the current translation unit. Only issue a warning.

Author: clementval

flang/lib/Lower/OpenACC.cpp

@@ -3295,9 +3295,12 @@ void Fortran::lower::finalizeOpenACCRoutineAttachment(
     mlir::func::FuncOp funcOp =
         mod.lookupSymbol<mlir::func::FuncOp>(mapping.first);
     if (!funcOp)
-      llvm::report_fatal_error(
-          "could not find function to attach OpenACC routine information.");
-    attachRoutineInfo(funcOp, mapping.second);
+      mlir::emitWarning(mod.getLoc(),
+                        llvm::Twine("function '") + llvm::Twine(mapping.first) +
+                            llvm::Twine("' in acc routine directive is not "
+                                        "found in this translation unit."));
+    else
+      attachRoutineInfo(funcOp, mapping.second);
   }
   accRoutineInfos.clear();
 }