Actually delay processing DelayedDllExportClasses until the outermost class is finished (PR40006)

This was already the intention of DelayedDllExportClasses, but code such as
this would break it:

template<typename> struct Tmpl {};
struct Outer {
    struct Inner {
        __declspec(dllexport) Inner() = default;
        unsigned int x = 0;
    };
    Tmpl<Inner> y;
};

ActOnFinishCXXNonNestedClass() would get called when the instantiation of
Templ<Inner> is finished, even though the compiler is still not finished with
Outer, causing the compile fail.

This hooks into Sema::{Push,Pop}ParsingClass() to avoid calling
ActOnFinishCXXNonNestedClass() for template instantiations while a class is
being parsed.

Differential revision: https://reviews.llvm.org/D70905
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index c6ffbfc..fe40932 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -3367,7 +3367,7 @@
 
     // We've finished parsing everything, including default argument
     // initializers.
-    Actions.ActOnFinishCXXNonNestedClass(TagDecl);
+    Actions.ActOnFinishCXXNonNestedClass();
   }
 
   if (TagDecl)