[clang-tools-extra] [clang-tidy] Add modernize-nlohmann-json-explicit-conversions check (PR #126425)

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

I have some mixed feelings as well, this could open the door for clang-tidy becoming a tool for modernizing "any" 3rd-party library usage. It feels like instead nlohmann/json should be the one responsible for providing a "migration path" / tooling for these types of deprecated features. 

It could also easily become a mess when we have to deal with / support different versions of the libraries. 

Thus I would vote for keeping clang-tidy as a general tool for C++/STL only. Perhaps these types of utilities could be implemented as a clang-tidy **plugin** in the respective 3rd-party repository?

https://github.com/llvm/llvm-project/pull/126425