[flang][openacc] Add semantic checks for atomic constructs (#149579)

An error report of the following code generating non-atomic code led us
to realize there are missing checks in the OpenACC atomics code. Add
some of those checks for atomic and sketch how the rest of the code
should proceed in checking the rest of the properties. The following
cases are all reported as errors.
```fortran
! Originally reported error!
!$acc atomic capture
a = b
c = b
!$acc end atomic capture
! Other ambiguous, but related errors!
!$acc atomic capture
x = i
i = x
!$acc end atomic capture
!$acc atomic capture
a = b
b = b
!$acc end atomic capture
!$acc atomic capture
a = b
a = c
!$acc end atomic capture
```