[Cygwin] Template instantiations should be exported by default (llvm#139799)

Behaves as same as both of Clang and GCC targetting MinGW. Required for
compatibility for Cygwin-GCC.

Divided from llvm#138773

Author: kikairoya

clang/lib/Sema/SemaDeclCXX.cpp

@@ -6465,7 +6465,7 @@ void Sema::checkClassLevelDLLAttribute(CXXRecordDecl *Class) {
   // declarations, except in MinGW mode.
   if (ClassExported && !ClassAttr->isInherited() &&
       TSK == TSK_ExplicitInstantiationDeclaration &&
-      !Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) {
+      !Context.getTargetInfo().getTriple().isOSCygMing()) {
     Class->dropAttr<DLLExportAttr>();
     return;
   }

clang/lib/Sema/SemaTemplate.cpp

@@ -8935,8 +8935,7 @@ Sema::CheckSpecializationInstantiationRedecl(SourceLocation NewLoc,
         // The declaration itself has not actually been instantiated, so it is
         // still okay to specialize it.
         StripImplicitInstantiation(
-            PrevDecl,
-            Context.getTargetInfo().getTriple().isWindowsGNUEnvironment());
+            PrevDecl, Context.getTargetInfo().getTriple().isOSCygMing());
         return false;
       }
       // Fall through
@@ -9804,7 +9803,7 @@ DeclResult Sema::ActOnExplicitInstantiation(
                                        : TSK_ExplicitInstantiationDeclaration;
 
   if (TSK == TSK_ExplicitInstantiationDeclaration &&
-      !Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) {
+      !Context.getTargetInfo().getTriple().isOSCygMing()) {
     // Check for dllexport class template instantiation declarations,
     // except for MinGW mode.
     for (const ParsedAttr &AL : Attr) {
@@ -9869,7 +9868,7 @@ DeclResult Sema::ActOnExplicitInstantiation(
     = PrevDecl ? PrevDecl->getTemplateSpecializationKind() : TSK_Undeclared;
 
   if (TSK == TSK_ExplicitInstantiationDefinition && PrevDecl != nullptr &&
-      Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) {
+      Context.getTargetInfo().getTriple().isOSCygMing()) {
     // Check for dllexport class template instantiation definitions in MinGW
     // mode, if a previous declaration of the instantiation was seen.
     for (const ParsedAttr &AL : Attr) {
@@ -10038,7 +10037,7 @@ DeclResult Sema::ActOnExplicitInstantiation(
     // In MinGW mode, export the template instantiation if the declaration
     // was marked dllexport.
     if (PrevDecl_TSK == TSK_ExplicitInstantiationDeclaration &&
-        Context.getTargetInfo().getTriple().isWindowsGNUEnvironment() &&
+        Context.getTargetInfo().getTriple().isOSCygMing() &&
         PrevDecl->hasAttr<DLLExportAttr>()) {
       dllExportImportClassTemplateSpecialization(*this, Def);
     }