[flang][NFC] Replace hardcoded attribute name

Replace the hardcoded attribute name with the constexpr StringRef
defined in the FIROps.td file.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D119422

Author: clementval

flang/lib/Optimizer/Dialect/FIROps.cpp

@@ -603,7 +603,8 @@ static void printCallOp(mlir::OpAsmPrinter &p, fir::CallOp &op) {
   else
     p << op.getOperand(0);
   p << '(' << op->getOperands().drop_front(isDirect ? 0 : 1) << ')';
-  p.printOptionalAttrDict(op->getAttrs(), {"callee"});
+  p.printOptionalAttrDict(op->getAttrs(),
+                          {fir::CallOp::getCalleeAttrNameStr()});
   auto resultTypes{op.getResultTypes()};
   llvm::SmallVector<Type> argTypes(
       llvm::drop_begin(op.getOperandTypes(), isDirect ? 0 : 1));
@@ -620,7 +621,8 @@ static mlir::ParseResult parseCallOp(mlir::OpAsmParser &parser,
   mlir::SymbolRefAttr funcAttr;
   bool isDirect = operands.empty();
   if (isDirect)
-    if (parser.parseAttribute(funcAttr, "callee", attrs))
+    if (parser.parseAttribute(funcAttr, fir::CallOp::getCalleeAttrNameStr(),
+                              attrs))
       return mlir::failure();
 
   Type type;