Argument Clinic bugfix: Don't let the C preprocessor "Monitor" see lines

that we are scanning for the output marker.  If we don't find it, we will
scan them again, so it sees them twice, and it can get confused (like
thinking we're still in a comment).

Tools/clinic/clinic.py

@@ -1252,9 +1252,10 @@ class BlockParser:
         match = self.start_re.match(line.lstrip())
         return match.group(1) if match else None
 
-    def _line(self):
+    def _line(self, lookahead=False):
         self.line_number += 1
         line = self.input.pop()
+        if not lookahead:
             self.language.parse_line(line)
         return line
 
@@ -1311,7 +1312,7 @@ class BlockParser:
         output_add, output_output = text_accumulator()
         arguments = None
         while self.input:
-            line = self._line()
+            line = self._line(lookahead=True)
             match = checksum_re.match(line.lstrip())
             arguments = match.group(1) if match else None
             if arguments: