[openacc] Add implicit flag to declare attribute

The declare attribute has been updated to allow implicit flag. This is
useful for variables that can be declare'd implicitly - like global
constants. The verifier has been updated to ensure that an implicit
declare'd variable has an implicit data action. The builder doesn't
require for this flag to be set so any code creating this attribute
will continue to work as-is.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D159124