[flang][OpenMP] Semantic checks for context selectors (llvm#123243)

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.

---------

Co-authored-by: Tom Eccles <tom.eccles@arm.com>

Author: kparzysz

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

@@ -3572,6 +3572,7 @@ 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,
@@ -3596,6 +3597,7 @@ 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 &&) {