Swift: fix stub revert prevention

It turns out the threshold of 5 lines for stub modification detection
was too strict: in case of a long class name the QL formatter will put
the closing brace of the empty class definition on a new line, leading
to codegen fail with an error thinking the stub was modified.

On the other side of things, also adding a base to a stub class was not
being detected as a modification.

Now the modification test is slightly smarter. If the stub still marked
as generated and

* has more than 6 lines, or
* the contents does not match a regexp aproximation of a plain stub

then codegen will abort. The test will still avoid reading the whole
contents of all the stubs.

swift/codegen/generators/qlgen.py

@@ -103,8 +103,8 @@ def _is_generated_stub(file, check_modification=False):
             # no lines
             return False
         if check_modification:
-            # one line already read, if we can read 4 other we are past the normal stub generation
-            line_threshold = 4
+            # one line already read, if we can read 5 other we are past the normal stub generation
+            line_threshold = 5
             if sum(1 for _ in zip(range(line_threshold), contents)) == line_threshold:
                 raise ModifiedStubMarkedAsGeneratedError(
                     f"{file.name} stub was modified but is still marked as generated")