[flang][extension] support concatenation with absent optional (#112678)

Fix #112593 by adding support in lowering to concatenation with an
absent optional _assumed length_ dummy argument because:
1. Most compilers seem to support it (most likely by accident).
2. This actually makes the compiler codegen simpler. Codegen was going
out of its way to poke the LLVM optimizer bear by producing an undef
argument for the length.

I insist on the fact that no compiler support this with _explicit
length_ optional arguments and the executable will segfault and I would
discourage users from using that "feature" because runtime checks for
bad optional dereference will kick when used (For instance, "nagfor
-C=present" will produce an executable that abort with an error message
. Flang does not have such runtime check option so far).

Hence, I am not updating the Extensions.md document because this is not
something I think we should advertise.

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -3373,19 +3373,7 @@ struct AbsentOpConversion : public fir::FIROpConversion<fir::AbsentOp> {
   matchAndRewrite(fir::AbsentOp absent, OpAdaptor,
                   mlir::ConversionPatternRewriter &rewriter) const override {
     mlir::Type ty = convertType(absent.getType());
-    mlir::Location loc = absent.getLoc();
-
-    if (mlir::isa<fir::BoxCharType>(absent.getType())) {
-      auto structTy = mlir::cast<mlir::LLVM::LLVMStructType>(ty);
-      assert(!structTy.isOpaque() && !structTy.getBody().empty());
-      auto undefStruct = rewriter.create<mlir::LLVM::UndefOp>(loc, ty);
-      auto nullField =
-          rewriter.create<mlir::LLVM::ZeroOp>(loc, structTy.getBody()[0]);
-      rewriter.replaceOpWithNewOp<mlir::LLVM::InsertValueOp>(
-          absent, undefStruct, nullField, 0);
-    } else {
-      rewriter.replaceOpWithNewOp<mlir::LLVM::ZeroOp>(absent, ty);
-    }
+    rewriter.replaceOpWithNewOp<mlir::LLVM::ZeroOp>(absent, ty);
     return mlir::success();
   }
 };