[Flang][OpenMP][Parser] Add clause list source location to REQUIRES directive

[skatrak]

Summary

This patch fixes a problem in the representation of the clause list for the REQUIRES directive. Location information was not present, making error messages not very descriptive.

This is patch 1/5 of a series splitting D149337 to simplify review.

[kiranchandramohan]

LG.

flang/lib/Parser/openmp-parsers.cpp

 
 // 2.8.2 Declare Simd construct
 TYPE_PARSER(
     sourced(construct<OpenMPDeclareSimdConstruct>(verbatim("DECLARE SIMD"_tok),
         maybe(parenthesized(name)), Parser<OmpClauseList>{})))
 
 // 2.4 Requires construct
 TYPE_PARSER(sourced(construct<OpenMPRequiresConstruct>(
-    verbatim("REQUIRES"_tok), some(Parser<OmpClause>{} / maybe(","_tok)))))
+    verbatim("REQUIRES"_tok), Parser<OmpClauseList>{})))
 
 // 2.15.2 Threadprivate directive
 TYPE_PARSER(sourced(construct<OpenMPThreadprivate>(
     verbatim("THREADPRIVATE"_tok), parenthesized(Parser<OmpObjectList>{}))))
 
 // 2.11.3 Declarative Allocate directive
 TYPE_PARSER(
     sourced(construct<OpenMPDeclarativeAllocate>(verbatim("ALLOCATE"_tok),

flang/test/Semantics/OpenMP/requires.f90

 ! RUN: %python %S/../test_errors.py %s %flang -fopenmp
 
-!$omp requires reverse_offload
+!$omp requires reverse_offload unified_shared_memory
+
+!ERROR: NOWAIT clause is not allowed on the REQUIRES directive
+!$omp requires nowait
 end