[flang] Fix some memory leaks (#121050)

This commit fixes some but not all memory leaks in Flang. There are
still 91 tests that fail with ASAN.

- Use `mlir::OwningOpRef` instead of `std::unique_ptr`. The latter does
not free allocations of nested blocks.
- Pass `ModuleOp` as value instead of reference.
- Add few missing deallocations in test cases and other places.

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -3087,7 +3087,7 @@ struct GlobalOpConversion : public fir::FIROpConversion<fir::GlobalOp> {
 private:
   static void addComdat(mlir::LLVM::GlobalOp &global,
                         mlir::ConversionPatternRewriter &rewriter,
-                        mlir::ModuleOp &module) {
+                        mlir::ModuleOp module) {
     const char *comdatName = "__llvm_comdat";
     mlir::LLVM::ComdatOp comdatOp =
         module.lookupSymbol<mlir::LLVM::ComdatOp>(comdatName);