[flang][OpenMP] Reorganize ALLOCATE-related semantic checks (#165719)

For ALLOCATORS and executable ALLOCATE first perform list item checks in
the context of an individual ALLOCATE clause or directive respectively,
then perform "global" checks, e.g. whether all list items are present on
the ALLOCATE statement.

These changes allowed to simplify the checks for presence on ALLOCATE
statement and the use of a predefined allocator.

Additionally, allow variable list item lists to be empty, add a test for
the related spec restriction.

This is a first step towards unifying OpenMPDeclarativeAllocate and
OpenMPExecutableAllocate into a single directive.

Author: kparzysz

flang/include/flang/Parser/parse-tree.h

@@ -5155,7 +5155,7 @@ struct OpenMPThreadprivate {
 struct OpenMPDeclarativeAllocate {
   TUPLE_CLASS_BOILERPLATE(OpenMPDeclarativeAllocate);
   CharBlock source;
-  std::tuple<Verbatim, OmpObjectList, OmpClauseList> t;
+  std::tuple<Verbatim, std::optional<OmpObjectList>, OmpClauseList> t;
 };
 
 struct OpenMPDeclarativeConstruct {

flang/include/flang/Semantics/openmp-utils.h

@@ -72,6 +72,8 @@ const parser::OmpObject *GetArgumentObject(const parser::OmpArgument &argument);
 bool IsCommonBlock(const Symbol &sym);
 bool IsExtendedListItem(const Symbol &sym);
 bool IsVariableListItem(const Symbol &sym);
+bool IsTypeParamInquiry(const Symbol &sym);
+bool IsStructureComponent(const Symbol &sym);
 bool IsVarOrFunctionRef(const MaybeExpr &expr);
 
 bool IsMapEnteringType(parser::OmpMapType::Value type);

flang/lib/Parser/openmp-parsers.cpp

@@ -2045,11 +2045,12 @@ TYPE_PARSER(sourced(construct<OpenMPCriticalConstruct>(
     OmpBlockConstructParser{llvm::omp::Directive::OMPD_critical})))
 
 // 2.11.3 Executable Allocate directive
-TYPE_PARSER(
-    sourced(construct<OpenMPExecutableAllocate>(verbatim("ALLOCATE"_tok),
-        maybe(parenthesized(Parser<OmpObjectList>{})), Parser<OmpClauseList>{},
-        maybe(nonemptyList(Parser<OpenMPDeclarativeAllocate>{})) / endOmpLine,
-        statement(allocateStmt))))
+TYPE_PARSER(sourced(construct<OpenMPExecutableAllocate>(
+    verbatim("ALLOCATE"_tok), maybe(parenthesized(Parser<OmpObjectList>{})),
+    Parser<OmpClauseList>{},
+    maybe(nonemptyList(startOmpLine >> Parser<OpenMPDeclarativeAllocate>{})) /
+        endOmpLine,
+    statement(allocateStmt))))
 
 // 2.8.2 Declare Simd construct
 TYPE_PARSER(sourced(construct<OpenMPDeclareSimdConstruct>(
@@ -2079,7 +2080,8 @@ TYPE_PARSER(sourced( //
 // 2.11.3 Declarative Allocate directive
 TYPE_PARSER(
     sourced(construct<OpenMPDeclarativeAllocate>(verbatim("ALLOCATE"_tok),
-        parenthesized(Parser<OmpObjectList>{}), Parser<OmpClauseList>{})) /
+        maybe(parenthesized(Parser<OmpObjectList>{})),
+        Parser<OmpClauseList>{})) /
     lookAhead(endOmpLine / !statement(allocateStmt)))
 
 // Assumes Construct

flang/lib/Parser/unparse.cpp

@@ -2502,9 +2502,7 @@ class UnparseVisitor {
   void Unparse(const OpenMPDeclarativeAllocate &x) {
     BeginOpenMP();
     Word("!$OMP ALLOCATE");
-    Put(" (");
-    Walk(std::get<OmpObjectList>(x.t));
-    Put(")");
+    Walk(" (", std::get<std::optional<OmpObjectList>>(x.t), ")");
     Walk(std::get<OmpClauseList>(x.t));
     Put("\n");
     EndOpenMP();