message: Fix build breakage on gcc-5
  Avoid `error: could not convert ?(const char*)"reduction"? from ?const char*? to ?llvm::StringLiteral?`. Tested with gcc-5.5.
diff: @@ -33,23 +33,23 @@ namespace mlir {
  /// Attribute name for the AffineArrayAttr which encodes the relationship
  /// between a structured op iterators' and its operands.
  static constexpr StringLiteral getIndexingMapsAttrName() {
  - return "indexing_maps";
  + return StringLiteral("indexing_maps");
  }
  /// Attribute name for the StrArrayAttr which encodes the type of a structured
  /// op's iterators.
  static constexpr StringLiteral getIteratorTypesAttrName() {
  - return "iterator_types";
  + return StringLiteral("iterator_types");
  }
  /// Use to encode that a particular iterator type has parallel semantics.
  inline static constexpr StringLiteral getParallelIteratorTypeName() {
  - return "parallel";
  + return StringLiteral("parallel");
  }
  /// Use to encode that a particular iterator type has reduction semantics.
  inline static constexpr StringLiteral getReductionIteratorTypeName() {
  - return "reduction";
  + return StringLiteral("reduction");
  }
  } // end namespace mlir
  