[mlir] Set intial value for integer FieldParser function

GCC complains FieldParser returns the value that does not have an
obvious initial value. Set an initial return value for the FieldParser
function.

The change is a no-op functionality-wise. The return value should be
updated within the function.

This should fix #64312

Differential Revision: https://reviews.llvm.org/D156848

Author: hcindyl

Diff for: mlir/include/mlir/IR/DialectImplementation.h

@@ -87,7 +87,7 @@ template <typename IntT>
 struct FieldParser<IntT,
                    std::enable_if_t<std::is_integral<IntT>::value, IntT>> {
   static FailureOr<IntT> parse(AsmParser &parser) {
-    IntT value;
+    IntT value = 0;
     if (parser.parseInteger(value))
       return failure();
     return value;