Split some code out from MergeAttributes()

Split some code out from MergeAttributes()

- Separate function to merge a child attribute into matching inherited
  attribute: The logic to merge a child attribute into matching
  inherited attribute in MergeAttribute() is only applicable to
  regular inheritance child.  The code is isolated and coherent enough
  that it can be separated into a function of its own.

- Separate function to merge next parent attribute: Partitions inherit
  from only a single parent.  The logic to merge an attribute from the
  next parent into the corresponding attribute inherited from previous
  parents in MergeAttribute() is only applicable to regular
  inheritance children.  This code is isolated enough that it can be
  separate into a function by itself.

These separations makes MergeAttribute() more readable by making it
easier to follow high level logic without getting entangled into
details.

Author: Ashutosh Bapat 
Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da@eisentraut.org