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

Add the "nowait" clause to the list of allowed clauses for the "workshare"
directive. This will make it consistent with other directives (which are
shared between C/C++ and Fortran).

The parser will still reject "nowait" on "!$omp workshare", so this has no
effect on accepting/rejecting Fortran source code.