ALways mark the entry of generator expression functions as used. Because, why...

ALways mark the entry of generator expression functions as used. Because, why not, they probably are. And it avoids false warnings.
Fixes https://github.com/cython/cython/issues/1699

Cython/Compiler/Nodes.py

@@ -4156,6 +4156,10 @@ class GeneratorBodyDefNode(DefNode):
                                 cname=cname, visibility='private')
         entry.func_cname = cname
         entry.qualified_name = EncodedString(self.name)
+        # Work-around for https://github.com/cython/cython/issues/1699
+        # We don't currently determine whether the generator entry is used or not,
+        # so mark it as used to avoid false warnings.
+        entry.used = True
         self.entry = entry
 
     def analyse_declarations(self, env):