[mlir][Transforms][WIP] Dialect conversion: Make materializations optional

Build all source/target/argument materializations after the conversion has succeeded. Provide a new configuration option for users to opt out of all automatic materializations. In that case, the resulting IR will have `builtin.unrealized_conversion_cast` ops.

Author: matthias-springer

mlir/include/mlir/Transforms/DialectConversion.h

@@ -1124,6 +1124,17 @@ struct ConversionConfig {
   // already been modified) and iterators into past IR state cannot be
   // represented at the moment.
   RewriterBase::Listener *listener = nullptr;
+
+  /// If set to "true", the dialect conversion attempts to build source/target/
+  /// argument materializations through the type converter API in lieu of
+  /// builtin.unrealized_conversion_cast ops. The conversion process fails if
+  /// at least one materialization could not be built.
+  ///
+  /// If set to "false", the dialect conversion does not does not build any
+  /// custom materializations and instead inserts
+  /// builtin.unrealized_conversion_cast ops to ensure that the resulting IR
+  /// is valid.
+  bool buildMaterializations = true;
 };
 
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