Bump src/third_party/reflect-cpp from 47111ba to 15f2c63 (#632)

Bumps [src/third_party/reflect-cpp](https://github.com/getml/reflect-cpp) from `47111ba` to `15f2c63`.
- [Release notes](https://github.com/getml/reflect-cpp/releases)
- [Commits](getml/reflect-cpp@47111ba...15f2c63)

---
updated-dependencies:
- dependency-name: src/third_party/reflect-cpp
  dependency-type: direct:production
...

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