[flang] Set isSigned=true for negative constant (NFC)

We're providing this as a negative signed value, so set the flag.
Currently doesn't make a difference, but will assert in the future.

Split out of #80309.

Author: nikic

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -1262,7 +1262,7 @@ struct EmboxCommonConversion : public fir::FIROpConversion<OP> {
       } else {
         auto maskAttr = mlir::IntegerAttr::get(
             rewriter.getIntegerType(8, /*isSigned=*/false),
-            llvm::APInt(8, (uint64_t)~_CFI_ADDENDUM_FLAG, /*isSigned=*/false));
+            llvm::APInt(8, (uint64_t)~_CFI_ADDENDUM_FLAG, /*isSigned=*/true));
         mlir::LLVM::ConstantOp mask = rewriter.create<mlir::LLVM::ConstantOp>(
             loc, rewriter.getI8Type(), maskAttr);
         extraField = rewriter.create<mlir::LLVM::AndOp>(loc, extraField, mask);