[flang] [llvm] [flang][OpenMP] Parse `bind` clause for `loop` direcitve. (PR #113662)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> For the first restriction, the nesting is defined on regions, so I think we cannot check that at compile time.

We should still do that for the cases that we can positively diagnose.  There is quite a bit of that happening in check-omp-structure.cpp.

https://github.com/llvm/llvm-project/pull/113662