[MLIR] Make ComplexType buildable if its element type is buildable

[cmtm]

Summary

If a ComplexType's element type is buildable, then that ComplexType should be
buildable. This is accomplished by the introduction of a new ODS class called
SameBuildabilityAs. This can be used by other types that are conditionally
buildable.

[cmtm]

Can you please submit this for me?
--author="Chris Morin <[e-mail]>"

mlir/include/mlir/IR/OpBase.td

 // diamond "inheritance".
 // TODO: we may extend this to a more general 'Buildable' trait, making some
 // Types and some Attrs buildable.
 class BuildableType<code builder> {
   // The builder call to invoke (if specified) to construct the BuildableType.
   code builderCall = builder;
 }
 
+// A type that's buildable iff the type passed as an argument is buildable.
+// This is intended for use by types like container types, which are only
+// buildable if the type of their elements is buildable.
+class SameBuildabilityAs<Type type, code builder> {
+  code builderCall = !if(!empty(type.builderCall), "", builder);
+}
+
 // Any type at all.
 def AnyType : Type<CPred<"true">, "any type">;
 
 // None type
 def NoneType : Type<CPred<"$_self.isa<::mlir::NoneType>()">, "none type",
                     "::mlir::NoneType">,
       BuildableType<"$_builder.getType<::mlir::NoneType>()">;
 

 
 class Complex<Type type>
     : Type<And<[
           CPred<"$_self.isa<::mlir::ComplexType>()">,
           SubstLeaves<"$_self",
                       "$_self.cast<::mlir::ComplexType>().getElementType()",
            type.predicate>]>,
            "complex type with " # type.description # " elements",
-           "::mlir::ComplexType"> {
+           "::mlir::ComplexType">,
+      SameBuildabilityAs<type, "::mlir::ComplexType::get($_builder.get" # type #
+                               "Type())"> {
   Type elementType = type;
 }
 
 def AnyComplex : Type<CPred<"$_self.isa<::mlir::ComplexType>()">,
                       "complex-type", "::mlir::ComplexType">;
 
 class OpaqueType<string dialect, string name, string description>
   : Type<CPred<"isOpaqueTypeWithName($_self, \""#dialect#"\", \""#name#"\")">,

mlir/test/mlir-tblgen/types.mlir

 func @complex_f64_tensor_success() {
   "test.complex_f64_tensor"() : () -> (tensor<complex<f64>>)
   return
 }
 
 // -----
 
 func @complex_f64_failure() {
-  // expected-error@+1 {{must be complex type with 64-bit float elements}}
+  // expected-error@+1 {{op inferred type incompatible with return type of operation}}
   "test.complex_f64"() : () -> (f64)
   return
 }
 
 // -----
 
 // CHECK-LABEL: @tuple_success
 func @tuple_success() {