Fix build issue with postgresql.

Execute_grouped_calculation is one of those places where
ActiveRecord forgets that it has ARel underpinnings, and
assumes that the values provided to group_values are
strings. This artificially hobbles otherwise functional
code. This patch stops assuming that incoming values
respond to to_sym, stops using string interpolation for
table aliases on objects that support aliasing, and stops
unnecessarily joining group_values on the relation.

Additionally, it calls to_sql, if available, on objects
sent to column_alias_for, in order to get a more reasonable
alias string than a non-string's default to_str method.

Ensure it works with mix of symbols and strings, and with a select
clause possibly containing more than one column.

Also remove support for pluck with an array of columns, in favor of
passing the list of attributes:

    Model.pluck(:a, :b)

See comments: https://github.com/rails/rails/pull/6500#issuecomment-6030292

Also use if/else block to not use short circuit return

Fix #5990

Thanks @splattael.

Add Changelog entry. Closes #4003

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

an alias is provided.  Without the alias, the column cannot be found
and properly typecast.

(and changed the use of true for a more database agnostic example).

Character classes are the specific regexp construct
to express alternation of individual characters.

Delete unnecessary line that was necessary before I changed the code and it became no longer necessary.

Make it the responsibility of the connection to hold onto an ARel visitor for generating SQL. This improves the code architecture generally, and solves some problems with marshalling. Adapter authors please take note: you now need to define an Adapter.visitor_for method, but it degrades gracefully with a deprecation warning for now.

In order make Relation behavior closer to Array
Made Relation#sum to accept block and delegate it to Array#sum

Ensure that calculations properly override the select value even if it's set in the default scope. Fixes #1395.