[MS] Defer dllexport inline friend functions like other inline methods

This special case was added in r264841, but the code breaks our
invariants by calling EmitTopLevelDecl without first creating a
HandlingTopLevelDeclRAII scope.

This fixes the PCH crash in https://crbug.com/884427. I was never able
to make a satisfactory reduction, unfortunately. I'm not very worried
about this regressing since this change makes the code simpler while
passing the existing test that shows we do emit dllexported friend
function definitions. Now we just defer their emission until the tag is
fully complete, which is generally good.

llvm-svn: 342516