Use min(max_heap_table_size, tmp_table_size) instead

per-file comments:
  sql/item_sum.cc
Bug#46018      group_concat(distinct ...) uses max_heap_table_size for memory allocations
    Item_sum_**::setup fixed, so they use ram_limitation() for Unique-s they embed

  sql/item_sum.h
Bug#46018      group_concat(distinct ...) uses max_heap_table_size for memory allocations
   Item_sum::ram_limitation() declared