use ExpressionElementRole for case targets in case()

use ExpressionElementRole for case targets in case()

Fixed regression where the :func:`_sql.text` construct would no longer be
accepted as a target case in the "whens" list within a :func:`_sql.case`
construct. The regression appears related to an attempt to guard against
some forms of literal values that were considered to be ambiguous when
passed here; however, there's no reason the target cases shouldn't be
interpreted as open-ended SQL expressions just like anywhere else, and a
literal string or tuple will be converted to a bound parameter as would be
the case elsewhere.

Fixes: #7287
Change-Id: I75478adfa115f3292cb1362cc5b2fdf152b0ed6f