Add .C for valid C++ extensions

[aryx]

test plan:
see related semgrep PR

• I ran make setup && make to update the generated code after editing a .atd file (TODO: have a CI check)
• I made sure we're still backward compatible with old versions of the CLI.
  For example, the Semgrep backend need to still be able to consume data generated
  by Semgrep 1.17.0.
  See https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades

[github-actions]

Backwards compatibility summary:

Checking backward compatibility of semgrep_output_v1.atd against past version v1.29.0
Skipping v1.30.0 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
Skipping v1.31.0 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
Skipping v1.31.1 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
Skipping v1.31.2 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
Skipping v1.32.0 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.33.0
Skipping v1.33.1 because commit 8849e56ddb0977e38a120a6cfbd1c396eb6fa15e has already been checked
Skipping v1.33.2 because commit 8849e56ddb0977e38a120a6cfbd1c396eb6fa15e has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.34.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.37.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.38.0
Skipping v1.38.1 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
Skipping v1.38.2 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
Skipping v1.38.3 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.39.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.40.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.41.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.42.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.43.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.44.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.45.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.46.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.47.0
Skipping v1.48.0 because commit 278ed753e0c66b8bfc3f2d805fde53be022dd4b6 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.49.0
Skipping v1.50.0 because commit 857682f41eb09e0b330a247ff1adf3bfeaf9d9ca has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.52.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.53.0
Skipping v1.54.0 because commit 3b72d494260258497e796d094b1a4916501a6df1 has already been checked
Skipping v1.54.1 because commit 3b72d494260258497e796d094b1a4916501a6df1 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.54.2
Skipping v1.54.3 because commit 9f1c50383a9a9969e2fe7a5f9bff9ca0a7c837bb has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.55.0
Skipping v1.55.1 because commit 6dffeaa692153fd33b4f154fddaefde1f2f1ae27 has already been checked
Skipping v1.55.2 because commit 6dffeaa692153fd33b4f154fddaefde1f2f1ae27 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.56.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.57.0
Skipping v1.58.0 because commit 4cc11b00d411c02fc611aa8c78a336520438fb48 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.59.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.59.1