[dllexport] odr-use constexpr default args for constructor closures

InstantiateDefaultCtorDefaultArgs() is supposed to mark default
constructor args as odr-used, since those args will be used when
emitting the constructor closure.

However, constexpr vars were not getting odr-used since
DoMarkVarDeclReferenced() defers them in MaybeODRUseExprs, and the code
was calling CleanupVarDeclMarking() which discarded those uses instead
of processing them.

(This came up in Chromium, crbug.com/1312086)

Differential revision: https://reviews.llvm.org/D123405

Author: zmodem

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -821,7 +821,7 @@ void Sema::InstantiateDefaultCtorDefaultArgs(CXXConstructorDecl *Ctor) {
   for (unsigned I = 0; I != NumParams; ++I) {
     (void)CheckCXXDefaultArgExpr(Attr->getLocation(), Ctor,
                                    Ctor->getParamDecl(I));
-    DiscardCleanupsInEvaluationContext();
+    CleanupVarDeclMarking();
   }
 }
 

clang/test/CodeGenCXX/dllexport-ctor-closure.cpp

@@ -88,3 +88,10 @@ CtorClosureOutOfLine::CtorClosureOutOfLine(const HasImplicitDtor2 &v) {}
 // CHECK-LABEL: define linkonce_odr dso_local x86_thiscallcc void @"??1HasImplicitDtor1@@QAE@XZ"
 // CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_FCtorClosureOutOfLine@@QAEXXZ"
 // CHECK-LABEL: define linkonce_odr dso_local x86_thiscallcc void @"??1HasImplicitDtor2@@QAE@XZ"
+
+struct SomeStruct {};
+constexpr SomeStruct kConstexprStruct;
+struct __declspec(dllexport) ConstexprDefaultArg {
+  ConstexprDefaultArg(SomeStruct = kConstexprStruct) {}
+};
+// CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_FConstexprDefaultArg@@QAEXXZ"