Fix double constexpr in constructors (#55846)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55846

Changelog: [Internal]

When constructors are marked as "constexpr", doxygen will output them as a function of type "constexpr" with "is-constexpr=yes". This causes the script to duplicate the keyword.

This diff fixes that by explicitly covering this edge case.

Reviewed By: cipolleschi

Differential Revision: D94901507

fbshipit-source-id: 384513381a86ba2efe07f14fd5976216ea5a8572