Coerce to float for Float with all native decimal backends

Coerce to float for Float with all native decimal backends

The result processor for the :class:`.Float` type now unconditionally
runs values through the ``float()`` processor if the dialect
specifies that it also supports "native decimal" mode.  While most
backends will deliver Python ``float`` objects for a floating point
datatype, the MySQL backends in some cases lack the typing information
in order to provide this and return ``Decimal`` unless the float
conversion is done.

Change-Id: I638f1480fb00a507036efaf0e0080f26893d98ad
Fixes: #4020