[mlir] Allow accessing TypeID of attrs/types defs

[math-fehr]

Summary

SelfOwningTypeID was inherited privately, so it was not possible to access
the TypeID of an attribute or a type definition.

[math-fehr]

Also, I do not have commit access, so would it be possible to land it for me if this is accepted?

mlir/include/mlir/IR/ExtensibleDialect.h

 // Dynamic attribute
 //===----------------------------------------------------------------------===//
 
 /// The definition of a dynamic attribute. A dynamic attribute is an attribute
 /// that is defined at runtime, and that can be registered at runtime by an
 /// extensible dialect (a dialect inheriting ExtensibleDialect). This class
 /// stores the parser, the printer, and the verifier of the attribute. Each
 /// dynamic attribute definition refers to one instance of this class.
-class DynamicAttrDefinition : SelfOwningTypeID {
+class DynamicAttrDefinition : public SelfOwningTypeID {
 public:
   using VerifierFn = llvm::unique_function<LogicalResult(
       function_ref<InFlightDiagnostic()>, ArrayRef<Attribute>) const>;
   using ParserFn = llvm::unique_function<ParseResult(
       AsmParser &parser, llvm::SmallVectorImpl<Attribute> &parsedAttributes)
                                              const>;
   using PrinterFn = llvm::unique_function<void(
       AsmPrinter &printer, ArrayRef<Attribute> params) const>;

 // Dynamic type
 //===----------------------------------------------------------------------===//
 
 /// The definition of a dynamic type. A dynamic type is a type that is
 /// defined at runtime, and that can be registered at runtime by an
 /// extensible dialect (a dialect inheriting ExtensibleDialect). This class
 /// stores the parser, the printer, and the verifier of the type. Each dynamic
 /// type definition refers to one instance of this class.
-class DynamicTypeDefinition : SelfOwningTypeID {
+class DynamicTypeDefinition : public SelfOwningTypeID {
 public:
   using VerifierFn = llvm::unique_function<LogicalResult(
       function_ref<InFlightDiagnostic()>, ArrayRef<Attribute>) const>;
   using ParserFn = llvm::unique_function<ParseResult(
       AsmParser &parser, llvm::SmallVectorImpl<Attribute> &parsedAttributes)
                                              const>;
   using PrinterFn = llvm::unique_function<void(
       AsmPrinter &printer, ArrayRef<Attribute> params) const>;