Fix assert on the variable which is used in omp clause is not marked as used

[jyu2]

Summary

The problem only happens with constexpr variable, for constexpr variable,
variable is not marked during parser expression. This is because compiler
might find some var's associate expressions may not actully an odr-used
later, the Var get kept in MaybeODRUseExprs, in normal case, at end of
process fullExpr, the variable will be mark during the call to
CleanupVarDeclMarking(). Since we are processing expression of OpenMP
clauses, and the ActOnFinishFullExpr is not getting called that casue
variable is not get marked.

One way to fix this is to call CleanupVarDeclMarking() at in EndOpenMPClause
for each omp directive.

This to fix https://bugs.llvm.org/show_bug.cgi?id=50206

[ABataev]

LG

clang/lib/Sema/SemaOpenMP.cpp

 }
 
 void Sema::StartOpenMPClause(OpenMPClauseKind K) {
   DSAStack->setClauseParsingMode(K);
 }
 
 void Sema::EndOpenMPClause() {
   DSAStack->setClauseParsingMode(/*K=*/OMPC_unknown);
+  CleanupVarDeclMarking();
 }
 
 static std::pair<ValueDecl *, bool>
 getPrivateItem(Sema &S, Expr *&RefExpr, SourceLocation &ELoc,
                SourceRange &ERange, bool AllowArraySection = false);
 
 /// Check consistency of the reduction clauses.
 static void checkReductionClauses(Sema &S, DSAStackTy *Stack,

clang/test/OpenMP/constexpr_capture.cpp

 // RUN: %clang_cc1 -verify -fopenmp -x c++ -triple powerpc64le-unknown-linux -S -emit-llvm %s -o - -std=c++11 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -verify -fopenmp -x c++  -fopenmp-targets=x86_64-pc-linux-gnu -triple powerpc64le-unknown-linux -S -emit-llvm %s -o - -std=c++11 2>&1 | FileCheck %s
 // expected-no-diagnostics
 
 template <int __v> struct integral_constant {
   static constexpr int value = __v;
 };
 
 template <typename _Tp, int v = 0, bool _IsArray = integral_constant<v>::value>
 struct decay {
   typedef int type;
 };
 struct V {
   template <typename TArg0 = int, typename = typename decay<TArg0>::type> V();
 };
+
+constexpr double h_chebyshev_coefs[] = {
+    1.0000020784639703, 0.0021491446496202074};
+
+void test(double *d_value)
+{
+#pragma omp target map(tofrom                          \
+                       : d_value [0:1]) map(always, to \
+                                            : h_chebyshev_coefs [0:2])
+  *d_value = h_chebyshev_coefs[1];  return;
+}
+
+// CHECK: void @__omp_offloading_{{.+}}test{{.+}}(double* %0)
+
 int main() {
 #pragma omp target
   V v;
   return 0;
 }
 
-// CHECK: call void @__omp_offloading_{{.+}}_main_l16()
+// CHECK: call void @__omp_offloading_{{.+}}_main_{{.+}}()