[Sema] Use MSVC inner class behavior on Itanium

[smeenai]

Summary

Windows Itanium aims to use MSVC export and import semantics. Inner
class members shouldn't be exported on a dllexport explicit
instantiation definition of the outer class, and they shouldn't be
imported on a dllimport explicit instantiation declaration of the outer
class (instead a local copy should be emitted). We were doing the first
but not the second, and this mismatch can lead to link errors. Fix the
behavior and add tests for both.

lib/Sema/SemaTemplateInstantiate.cpp

if (auto *Function = dyn_cast<FunctionDecl>(D)) {
       
       MemberSpecializationInfo *MSInfo = Record->getMemberSpecializationInfo();
       assert(MSInfo && "No member specialization information?");
       
       if (MSInfo->getTemplateSpecializationKind()
                                                 == TSK_ExplicitSpecialization)
         continue;
 
-      if (Context.getTargetInfo().getCXXABI().isMicrosoft() &&
+      if ((Context.getTargetInfo().getCXXABI().isMicrosoft() ||
+           Context.getTargetInfo().getTriple().isWindowsItaniumEnvironment()) &&
           TSK == TSK_ExplicitInstantiationDeclaration) {
-        // In MSVC mode, explicit instantiation decl of the outer class doesn't
-        // affect the inner class.
+        // In MSVC and Windows Itanium mode, explicit instantiation decl of the
+        // outer class doesn't affect the inner class.
         continue;
       }
 
       if (CheckSpecializationInstantiationRedecl(PointOfInstantiation, TSK, 
                                                  Record, 
                                         MSInfo->getTemplateSpecializationKind(),
                                               MSInfo->getPointOfInstantiation(),
                                                  SuppressNew) ||

test/CodeGenCXX/windows-itanium-dllexport.cpp

 // RUN: %clang_cc1 -emit-llvm -triple i686-windows-itanium -fdeclspec %s -o - | FileCheck %s
 
+#define JOIN2(x, y) x##y
+#define JOIN(x, y) JOIN2(x, y)
+#define UNIQ(name) JOIN(name, __LINE__)
+#define USEMEMFUNC(class, func) void (class::*UNIQ(use)())() { return &class::func; }
+
 struct __declspec(dllexport) s {
   void f() {}
 };
 
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1saSERKS_
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1s1fEv
 
 template <class T>

 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIcEaSERKS0_
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIcE1fEv
 
 c<double> g;
 template class __declspec(dllexport) c<double>;
 
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIdEaSERKS0_
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIdE1fEv
+
+template <class T>
+struct outer {
+  void f() {}
+  struct inner {
+    void f() {}
+  };
+};
+
+template class __declspec(dllexport) outer<int>;
+
+// CHECK: define {{.*}} dllexport {{.*}} @_ZN5outerIiE1fEv
+// CHECK-NOT: define {{.*}} dllexport {{.*}} @_ZN5outerIiE5inner1fEv
+
+extern template class __declspec(dllimport) outer<char>;
+USEMEMFUNC(outer<char>, f)
+USEMEMFUNC(outer<char>::inner, f)
+
+// CHECK: declare dllimport {{.*}} @_ZN5outerIcE1fEv
+// CHECK: define {{.*}} @_ZN5outerIcE5inner1fEv