First of all, you'd need to somehow make the Free Pascal and whatever C++ you choose to talk to each other, the C++ compiler to be able to instantiate Free Pascal classes (so you can use LCL) and the Free Pascal linker (in the platforms where it is used) to be able to link against C++ objects. Also it would need the C++ compiler to take into account the minute details of Free Pascal's semantics (e.g. in C/C++ it is undefined behavior to dereference a NULL object, but in Free Pascal it is perfectly valid - it'll just do whatever the underlying system does when someone tries to access memory at address 0 - so the C/C++ compiler should not assume that never happens).
This is the hardest part, but the second hardest part isn't that easy either: you'd also need to extend CodeTools (the framework that Lazarus uses to parse and maintain an in-memory view of the source code you are editing, the units it uses and their structure and basically keep a somewhat mirrored - but mutable - view of what the compiler would see, plus the extra stuff an IDE needs like the ability to perform queries to find symbols, declarations, etc and manipulations for refactoring) to understand and work with C++ code, so that IDE functionality for updating the code when you modify a form, create an event handler, syntax and word completion, code completion (e.g. declaring a property and automatically creating the backing field and setter method or implementing abstract/virtual methods from a superclass - that in this case might be in Free Pascal instead of C++), etc will work.
Note that Embarcadero (who develop Delphi and C++ Builder) did the compiler part of the above using Clang for their C++ Builder, but since Clang doesn't require sharing the changes, that effort was basically lost (only usable by those who are still stuck using Embarcadero products, until the project inevitably dies as C++ Builder was far less popular than Delphi), so anyone who wouldn't like to do something similar with Lazarus would have to start from scratch - an unnecessary waste of time thanks to Clang choosing a permissive license.
In practice however i do not think this will happen because it'll need coordination between several teams - and each team to care about the others. For example, Embarcadero who did that with Clang are now stuck with an ancient version of it because of their modification. For it to be done right, it would need the Free Pascal developers to cooperate with the Clang developers to not break each others' code and for the Clang developers to accept the extensions the Free Pascal developers introduce to support the extra functionality that interfacing with Free Pascal needs.