[Sema] Use MS ABI behavior for dllexport in Itanium

[smeenai]

Summary

Similar to r284288, make the Itanium ABI follow MS ABI dllexport
semantics in the case of an explicit instantiation declaration followed
by a dllexport explicit instantiation definition.

lib/Sema/SemaTemplate.cpp

if (Def) {
     // TSK_ExplicitInstantiationDefinition
     if (Old_TSK == TSK_ExplicitInstantiationDeclaration &&
         (TSK == TSK_ExplicitInstantiationDefinition ||
          DLLImportExplicitInstantiationDef)) {
       // FIXME: Need to notify the ASTMutationListener that we did this.
       Def->setTemplateSpecializationKind(TSK);
 
       if (!getDLLAttr(Def) && getDLLAttr(Specialization) &&
-          Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+          (Context.getTargetInfo().getCXXABI().isMicrosoft() ||
+           Context.getTargetInfo().getTriple().isWindowsItaniumEnvironment())) {
         // In the MS ABI, an explicit instantiation definition can add a dll
         // attribute to a template with a previous instantiation declaration.
         // MinGW doesn't allow this.
         auto *A = cast<InheritableAttr>(
             getDLLAttr(Specialization)->clone(getASTContext()));
         A->setInherited(true);
         Def->addAttr(A);
 

test/CodeGenCXX/windows-itanium-dllexport.cpp

 // RUN: %clang_cc1 -emit-llvm -triple i686-windows-itanium -fdeclspec %s -o - | FileCheck %s
 
 struct __declspec(dllexport) s {
   void f() {}
 };
 
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1saSERKS_
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1s1fEv
 
+template <class T>
+class c {
+  void f() {}
+};
+
+template class __declspec(dllexport) c<int>;
+
+// CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIiEaSERKS0_
+// CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIiE1fEv
+
+extern template class c<char>;
+template class __declspec(dllexport) c<char>;
+
+// CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIcEaSERKS0_
+// CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIcE1fEv
+