[Flang][OpenMP] Deprecate Allocate Directive (#142378)
As part of OpenMP 5.2, the allocate directive has been deprecated in
favour of the allocators construct for Fortran when an ALLOCATE
statement follows the OpenMP allocate directive.
To enable this in flang, a warning has been added informing the user of
this. Tests to ensure this behaviour is continued are also included.
See also: #110008
