[flang] [llvm] [flang][OpenMP] Overhaul implementation of ATOMIC construct (PR #137852)

[Tom Eccles via llvm-commits]

https://github.com/tblah commented:

Thanks @kparzysz for the massive effort this patch must have taken to implement.

I have a few minor comments but I agree with your overall approach.

Right now I cannot build this with -Werror:
```
/home/$USER/llvm-project/flang/lib/Semantics/check-omp-structure.cpp:2820:56: error: missing field 'iff' initializer [-Werror,-Wmissing-field-initializers]
 2820 |             GetActionStmt(std::get<parser::Block>(s.t))};
      |                                                        ^
1 error generated.
ninja: build stopped: subcommand failed.

```

https://github.com/llvm/llvm-project/pull/137852