innobase_copy_index_field_def(): Remove parameter heap.  The field->name
will not be deallocated before the copied index has been added to the
data dictionary cache.

innobase_copy_index_def(): Add const qualifier to dict_index_t*.
Add an assertion !dict_index_is_clust(index).

ha_innobase::add_index(): Add the missing user_thd parameter to the
convert_error_code_to_mysql() call that was forgotten from r1686.

merge_index_field_struct: Add const qualifier to field_name.

merge_index_def_struct: Add const qualifier to name.

row_merge_build_indexes(): Improve the function comment.

row_merge_drop_table(): Remove bogus assertion ut_a(table->to_be_dropped).