Convert expression type for concat + Enum

Convert expression type for concat + Enum

Fixed bug involving new value translation and validation feature
in :class:`.Enum` whereby using the enum object in a string
concatenation would maintain the :class:`.Enum` type as the type
of the expression overall, producing missing lookups.  A string
concatenation against an :class:`.Enum`-typed column now uses
:class:`.String` as the datatype of the expression itself.

Change-Id: Id402054e3ef008e0250c740dbb7e1c80f339fe78
Fixes: #3833