alter the collation of string type for collate()

alter the collation of string type for collate()

Fixed issue where the :func:`_sql.collate` construct, which explicitly sets
a collation for a given expression, would maintain collation settings for
the underlying type object from the expression, causing SQL expressions to
have both collations stated at once when used in further expressions for
specific dialects that render explicit type casts, such as that of asyncpg.
The :func:`_sql.collate` construct now assigns its own type to explicitly
include the new collation, assuming it's a string type.

Fixes: #11576
Change-Id: I6fc8904d2bcbc21f11bbca57e4a451ed0edbd879