[flang] [llvm] [LLVM][OpenMP] Add "nowait" clause as valid for "workshare" (PR #88426)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> However, we probably want to add semantic check for having `nowait` on both `workshare` and `end workshare`. How about the `copyprivate` clause on the `single` directive?

The flang parser actually checks for such cases explicitly, and rejects cases of "nowait" on directives that allow it based on the .td contents, but where Fortran syntax requires them to be on the "end" directive.  I added "workshare" to the list, and updated the testcase.  I have checked that "copyprivate" on "!$omp single" is rejected as well.

In the current form this PR will not have any effect on the accepted syntax, it will simply allow "nowait" to be associated with "workshare" internally to the compiler.

https://github.com/llvm/llvm-project/pull/88426