cpp: do not mark boolean(isl_bool) constructor as explicit

cpp: do not mark boolean(isl_bool) constructor as explicit

The isl::boolean class was created with the goal of disallowing
construction from isl_bool and always requiring the user to convert an
isl_bool to isl:boolean using isl::manage(isl_bool). To achieve this the
boolean(isl_bool) constructor was marked private with the goal of
triggering compiler errors whenever an isl boolean object is constructed
from an isl_bool value. However, as the boolean(isl_bool) constructor
was accidentally marked as 'explicit', it did not take part in the
normal overload resolution. As a result, users did not receive an error
that a private constructor is accessed, but instead the isl_bool value
was implicitly converted to bool and the public boolean(bool)
constructor was called. This was incorrect, as the conversion from
isl_bool to bool turned isl_bool_error values silently into true. By
dropping the 'explicit' marker the boolean(isl_bool) constructor takes
again part in overload resolution and instead of silent corruptions, the
user receives the expected compiler error.

Suggested-by: Michael Kruse <isl@meinersbur.de>
Signed-off-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>