[OPENMP]Fix PR49790: Constexpr values not handled in `omp declare mapper` clause.

[ABataev]

Summary

Patch allows using of constexpr vars evaluatable to constant calue to be
used in declare mapper construct.

[jdoerfert]

LG

clang/lib/Sema/SemaOpenMP.cpp

Sema::ActOnOpenMPDeclareMapperDirectiveVarDecl(Scope *S, QualType MapperType,
   Expr *E = buildDeclRefExpr(*this, VD, MapperType, StartLoc);
   DSAStack->addDeclareMapperVarRef(E);
   return E;
 }
 
 bool Sema::isOpenMPDeclareMapperVarDeclAllowed(const VarDecl *VD) const {
   assert(LangOpts.OpenMP && "Expected OpenMP mode.");
   const Expr *Ref = DSAStack->getDeclareMapperVarRef();
-  if (const auto *DRE = cast_or_null<DeclRefExpr>(Ref))
-    return VD->getCanonicalDecl() == DRE->getDecl()->getCanonicalDecl();
+  if (const auto *DRE = cast_or_null<DeclRefExpr>(Ref)) {
+    if (VD->getCanonicalDecl() == DRE->getDecl()->getCanonicalDecl())
+      return true;
+    if (VD->isUsableInConstantExpressions(Context))
+      return true;
+    return false;
+  }
   return true;
 }
 
 const ValueDecl *Sema::getOpenMPDeclareMapperVarName() const {
   assert(LangOpts.OpenMP && "Expected OpenMP mode.");
   return cast<DeclRefExpr>(DSAStack->getDeclareMapperVarRef())->getDecl();
 }
 

clang/test/OpenMP/declare_mapper_ast_print.cpp

 // CHECK: #pragma omp declare mapper (id : N1::vec v) map(tofrom: v.len){{$}}
 // CHECK: #pragma omp declare mapper (id : dat::datin v) map(tofrom: v.in){{$}}
 // CHECK: };
 // CHECK: template<> class dat<double> {
 // CHECK: #pragma omp declare mapper (id : N1::vec v) map(tofrom: v.len){{$}}
 // CHECK: #pragma omp declare mapper (id : dat<double>::datin v) map(tofrom: v.in){{$}}
 // CHECK: };
 
-#pragma omp declare mapper(default : N1::vec kk) map(kk.len) map(kk.data[0:2])
-// CHECK: #pragma omp declare mapper (default : N1::vec kk) map(tofrom: kk.len) map(tofrom: kk.data[0:2]){{$}}
+constexpr int N = 2;
+#pragma omp declare mapper(default : N1::vec kk) map(kk.len) map(kk.data[0:N])
+// CHECK: #pragma omp declare mapper (default : N1::vec kk) map(tofrom: kk.len) map(tofrom: kk.data[0:N]){{$}}
 #pragma omp declare mapper(dat<double> d) map(to: d.d)
 // CHECK: #pragma omp declare mapper (default : dat<double> d) map(to: d.d){{$}}
 
 template <typename T>
 T foo(T a) {
   struct foodatchild {
     T k;
   };