[flang][OpenMP] Semantic checks for context selectors

This implements checks of the validity of context set selectors and
trait selectors, plus the types of trait properties. Clause properties
are also validated, but not name or extension properties.

Author: kparzysz

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

@@ -3569,6 +3569,7 @@ struct OmpTraitProperty {
 // Trait-set-selectors:
 //    [D]evice, [T]arget_device, [C]onstruct, [I]mplementation, [U]ser.
 struct OmpTraitSelectorName {
+  std::string ToString() const;
   CharBlock source;
   UNION_CLASS_BOILERPLATE(OmpTraitSelectorName);
   ENUM_CLASS(Value, Arch, Atomic_Default_Mem_Order, Condition, Device_Num,
@@ -3593,6 +3594,7 @@ struct OmpTraitSelector {
 //    CONSTRUCT | DEVICE | IMPLEMENTATION | USER |  // since 5.0
 //    TARGET_DEVICE                                 // since 5.1
 struct OmpTraitSetSelectorName {
+  std::string ToString() const;
   CharBlock source;
   ENUM_CLASS(Value, Construct, Device, Implementation, Target_Device, User)
   WRAPPER_CLASS_BOILERPLATE(OmpTraitSetSelectorName, Value);

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

@@ -72,6 +72,7 @@ DECLARE_DESCRIPTOR(parser::OmpAlignModifier);
 DECLARE_DESCRIPTOR(parser::OmpAllocatorComplexModifier);
 DECLARE_DESCRIPTOR(parser::OmpAllocatorSimpleModifier);
 DECLARE_DESCRIPTOR(parser::OmpChunkModifier);
+DECLARE_DESCRIPTOR(parser::OmpContextSelector);
 DECLARE_DESCRIPTOR(parser::OmpDependenceType);
 DECLARE_DESCRIPTOR(parser::OmpDeviceModifier);
 DECLARE_DESCRIPTOR(parser::OmpDirectiveNameModifier);

flang/lib/Parser/parse-tree.cpp

@@ -281,6 +281,26 @@ OmpTaskDependenceType::Value OmpDependClause::TaskDep::GetTaskDepType() const {
   }
 }
 
+std::string OmpTraitSelectorName::ToString() const {
+  return common::visit( //
+      common::visitors{
+          [&](Value v) { //
+            return std::string(EnumToString(v));
+          },
+          [&](llvm::omp::Directive d) {
+            return llvm::omp::getOpenMPDirectiveName(d).str();
+          },
+          [&](const std::string &s) { //
+            return s;
+          },
+      },
+      u);
+}
+
+std::string OmpTraitSetSelectorName::ToString() const {
+  return std::string(EnumToString(v));
+}
+
 } // namespace Fortran::parser
 
 template <typename C> static llvm::omp::Clause getClauseIdForClass(C &&) {