Fix collation handling for grouping keys in eager aggregation

Fix collation handling for grouping keys in eager aggregation

When determining if it is safe to use an expression as a grouping key
for partial aggregation, eager aggregation relies on the B-tree
equalimage support function to ensure that equality implies image
equality.

Previously, the code incorrectly passed the default collation of the
expression's data type to the equalimage procedure, rather than the
expression's actual collation.  As a result, if a column used a
non-deterministic collation but the base type's default collation was
deterministic, eager aggregation would incorrectly assume that the
column was safe for byte-level grouping.  This could cause rows to be
prematurely grouped and subsequently discarded by strict join
conditions, resulting in incorrect query results.

This patch fixes the issue by passing the expression's actual
collation to the equalimage procedure.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48A53PY1Y4zoj7YhxPww9fO1hfnbdntKfA855zpXfVFRA@mail.gmail.com