The fidlc code lives in //zircon/tools/fidl. Once build unification is complete, it can be moved here. In the meantime, this directory just contains the goldens.
The fidlc goldens are the result of running fidlc on the test libraries in //zircon/tools/fidl/testdata (see README.md there). In particular, we store golden files for the JSON IR and coding tables.
To regenerate, run fx regen-goldens fidlc.
To test, run fx test fidlc_golden_tests.