Add a utility method to MLIRContext get a registered dialect with the...

    Add a utility method to MLIRContext get a registered dialect with the derived type instead of the string name. The derived dialect type must provide a static 'getDialectNamespace' method.

    This means that we can now do something like:
      ctx->getRegisteredDialect<LLVMDialect>();

    as opposed to:
      static_cast<LLVMDialect *>(ctx->getRegisteredDialect("llvm");

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PiperOrigin-RevId: 247989896