Duplicated async statements in OpenACC

[schreiberx]

The async ACC clause results in duplicated async statements, which are fixed with this PR.

!$acc kernels default(present) async(1) async(1)
!$acc end kernels

.!$acc parallel default(present) async(1) async(1)
!$acc end parallel

.!$acc update if_present host(x) async(1) async(1)

.!$acc enter data copyin(x) async(1)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (1b5eb08) to head (260d03f).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3055      +/-   ##
==========================================
- Coverage   99.92%   99.92%   -0.01%     
==========================================
  Files         366      366              
  Lines       51678    51668      -10     
==========================================
- Hits        51638    51628      -10     
  Misses         40       40              

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[sergisiso]

@schreiberx Good spot, but we can actually go further and completely remove the _build_async_string. Since the async is in a PSyIR Clause it should be the PSyIR backend that produces the right code when visiting the node and not the the begin_string method. The method was probably a leftover of gen_code.

I took the liberty to do the change in this branch, but this means that the review will need to be finished by @arporter or @LonelyCat124

[arporter]

Thanks very much Martin and Sergi.
This looks good.
I've set the integration tests running. If they are green then this is good to go.

[arporter]

Integration tests were all green.
Will proceed to merge.