SQL Server is not native boolean; add new flag for CHECK constraint

SQL Server is not native boolean; add new flag for CHECK constraint

Fixed a 1.2 regression caused by :ticket:`4061` where the SQL Server
"BIT" type would be considered to be "native boolean".  The goal here
was to avoid creating a CHECK constraint on the column, however the bigger
issue is that the BIT value does not behave like a true/false constant
and cannot be interpreted as a standalone expression, e.g.
"WHERE <column>".   The SQL Server dialect now goes back to being
non-native boolean, but with an extra flag that still avoids creating
the CHECK constraint.

Change-Id: I4765d2a2a00b0d14f50282603cc4d48d4739dac1
Fixes: #4250