[flang][OpenMP] Make OmpDirectiveNameModifier a distrinct type

It was an alias for OmpDirectiveName, which could cause confusion in
parse-tree visitors: a visitor for OmpDirectiveNameModifier could be
executed for an OmpDirectiveName node, leading to unexpected results.

Author: kparzysz

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

@@ -3469,13 +3469,20 @@ WRAPPER_CLASS(PauseStmt, std::optional<StopCode>);
 
 // --- Common definitions
 
+#define INHERITED_WRAPPER_CLASS_BOILERPLATE(classname, basename) \
+  BOILERPLATE(classname); \
+  classname(decltype(basename::v) &&x) : basename(std::move(x)) {} \
+  classname(basename &&base) : basename(std::move(base)) {} \
+  using WrapperTrait = std::true_type
+
 struct OmpClause;
 struct OmpDirectiveSpecification;
 
 struct OmpDirectiveName {
   // No boilerplates: this class should be copyable, movable, etc.
   constexpr OmpDirectiveName() = default;
   constexpr OmpDirectiveName(const OmpDirectiveName &) = default;
+  constexpr OmpDirectiveName(llvm::omp::Directive x) : v(x) {}
   // Construct from an already parsed text. Use Verbatim for this because
   // Verbatim's source corresponds to an actual source location.
   // This allows "construct<OmpDirectiveName>(Verbatim("<name>"))".
@@ -3848,7 +3855,10 @@ struct OmpDeviceModifier {
 // [*] The IF clause is allowed on CANCEL in OpenMP 4.5, but only without
 // the directive-name-modifier. For the sake of uniformity CANCEL can be
 // considered a valid value in 4.5 as well.
-using OmpDirectiveNameModifier = OmpDirectiveName;
+struct OmpDirectiveNameModifier : public OmpDirectiveName {
+  INHERITED_WRAPPER_CLASS_BOILERPLATE(
+      OmpDirectiveNameModifier, OmpDirectiveName);
+};
 
 // Ref: [5.1:205-209], [5.2:166-168]
 //

flang/lib/Parser/openmp-parsers.cpp

@@ -466,6 +466,8 @@ TYPE_PARSER(construct<OmpDeviceModifier>(
     "ANCESTOR" >> pure(OmpDeviceModifier::Value::Ancestor) ||
     "DEVICE_NUM" >> pure(OmpDeviceModifier::Value::Device_Num)))
 
+TYPE_PARSER(construct<OmpDirectiveNameModifier>(OmpDirectiveNameParser{}))
+
 TYPE_PARSER(construct<OmpExpectation>( //
     "PRESENT" >> pure(OmpExpectation::Value::Present)))
 
@@ -609,7 +611,8 @@ TYPE_PARSER(sourced(construct<OmpFromClause::Modifier>(
 TYPE_PARSER(sourced(
     construct<OmpGrainsizeClause::Modifier>(Parser<OmpPrescriptiveness>{})))
 
-TYPE_PARSER(sourced(construct<OmpIfClause::Modifier>(OmpDirectiveNameParser{})))
+TYPE_PARSER(sourced(
+    construct<OmpIfClause::Modifier>(Parser<OmpDirectiveNameModifier>{})))
 
 TYPE_PARSER(sourced(
     construct<OmpInitClause::Modifier>(