[mlir][linalg] Add a builder for `linalg.(indexed_)generic`.

[pifon2a]

Format.

[nicolasvasilache]

Quick FMI, did you see the more sugared makeGenericLinalgOp version and its use here https://github.com/llvm/llvm-project/blob/19e5b2bccb4c93f85ca098528f9fa42344e8312f/mlir/lib/Dialect/Linalg/EDSC/Builders.cpp#L329 ?
I'd welcome and indexed version of this that does the same thing with additional IV insertion into blocks.

Unlike the other more controversial implicit builder pieces, this should be kosher enough to be used anywhere, it is just a better Linalg builder that uses more structured (pun intended) helper structs.

mlir/include/mlir/Dialect/Linalg/IR/LinalgStructuredOps.td

llvm::Optional<SmallVector<StringRef, 8>> referenceIterators() {
         "No such thing as reference iterator types for a generic op.");
      }
 
     llvm::Optional<SmallVector<AffineMap, 8>> referenceIndexingMaps() {
       llvm_unreachable(
         "No such thing as reference indexing maps for a generic op.");
      }
   }];
+
+  let builders = [
+    OpBuilder<"OpBuilder &builder, OperationState &result, "
+              "ArrayRef<Type> resultTypes, ValueRange args, "
+              "int64_t inputCount, int64_t outputCount, "
+              "ArrayRef<AffineMap> indexingMaps, "
+              "ArrayRef<StringRef> iteratorTypes", [{
+        return build(builder, result, resultTypes, args,
+                     builder.getI64IntegerAttr(inputCount),
+                     builder.getI64IntegerAttr(outputCount),
+                     builder.getAffineMapArrayAttr(indexingMaps),
+                     builder.getStrArrayAttr(iteratorTypes),
+                     /*doc=*/nullptr, /*library_call=*/nullptr);
+  }]>];
+
   let printer = [{ return ::print(p, *this); }];
   let parser = [{ return ::parseGenericOp(parser, result); }];
 }
 
 /// Index-free GenericOp.
 def GenericOp : GenericOpBase<"generic"> {
   let description = [{
     Generic Linalg op form where the key properties of the computation are