[flang][NFC] Replace hardcoded attribute name

[clementval]

Summary

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

[kiranchandramohan]

LGTM

flang/lib/Optimizer/Dialect/FIROps.cpp

static void printCallOp(mlir::OpAsmPrinter &p, fir::CallOp &op) {
   auto callee = op.callee();
   bool isDirect = callee.hasValue();
   p << ' ';
   if (isDirect)
     p << callee.getValue();
   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));
   p << " : " << FunctionType::get(op.getContext(), argTypes, resultTypes);
 }
 
 static mlir::ParseResult parseCallOp(mlir::OpAsmParser &parser,
                                      mlir::OperationState &result) {
   llvm::SmallVector<mlir::OpAsmParser::OperandType> operands;
   if (parser.parseOperandList(operands))
     return mlir::failure();
 
   mlir::NamedAttrList attrs;
   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;
   if (parser.parseOperandList(operands, mlir::OpAsmParser::Delimiter::Paren) ||
       parser.parseOptionalAttrDict(attrs) || parser.parseColon() ||
       parser.parseType(type))
     return mlir::failure();