innobase_create_temporary_tablename(): Add static qualifier.  Allocate
the memory from heap.  Return a fixed identifier.

ha_innobase::add_index(): Allocate all memory from a single heap.
Simplify error handling.  Use a fixed prefix for temporary table names,
because only one add_index() can be active for a given table.

mem_heap_empty_noninline(): New function, to be called from ha_innodb.cc.

row_build_index_for_mysql(): Remove the parameter new_primary.