message: Fix MacOS build: static constexpr must be defined
  This can be removed in C++17.
diff: @@ -31,6 +31,9 @@ using namespace mlir::detail;
  // Integer Type
  //===----------------------------------------------------------------------===//
  +// static constexpr must have a definition (until in C++17 and inline variable).
  +constexpr unsigned IntegerType::kMaxWidth;
  +
  /// Verify the construction of an integer type.
  LogicalResult IntegerType::verifyConstructionInvariants(
  llvm::Optional<Location> loc, MLIRContext *context, unsigned width) {
  