[Cygwin] Internal class in explicitly-instantiation-declarated template should be instantiated

In-code comment says "explicit instantiation decl of the outer class
doesn't affect the inner class" but this behavior seems MSVC
specific, mingw-gcc and cygwin-gcc does not.
Clang should honor gcc's behavior.

This change fixes std::string compatibilty and resolves strange link
error (statically linked), strange crash (dynamically linked) using
libstdc++ on Cygwin.

HACK! only change behavior for Cygwin, not MinGW, because linking to
libc++ relies on the old behavior, even though it differs from GCC.

Co-authored-by: Jeremy Drake <[email protected]>

Author: kikairoya

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -4361,6 +4361,7 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
         continue;
 
       if (Context.getTargetInfo().getTriple().isOSWindows() &&
+          !Context.getTargetInfo().getTriple().isWindowsCygwinEnvironment() &&
           TSK == TSK_ExplicitInstantiationDeclaration) {
         // On Windows, explicit instantiation decl of the outer class doesn't
         // affect the inner class. Typically extern template declarations are