[Flang][OpenMP][Lower] Program level implicit SAVE variable handling for declare target

This is an attempt at mimicing the method in which
threadprivate handles the following type of variables:

program main
  integer :: i
  !$omp declare target to(i)
end

Which essentially generates a GlobalOp for the variable (which
would normally only be an alloca) when it's instantiated. The
main difference is there is no operation generated within the
function, instead the declare target attribute is appended
later within handleDeclareTarget.

Reviewers: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D152037