Note that the Free Pascal/Lazarus equivalent to VCL is two libraries: FCL and LCL, with the former being part and developed by the developers of Free Pascal and the latter being part and developed by the developers of Lazarus (while there is an overlap, they are two different projects with different teams). FCL is also used by other projects (e.g. FPGUI uses FCL but not LCL) and if you install just Free Pascal you only get FCL, not LCL.

With that in mind and considering the context i guess you meant LCL.

Now, LCL would be very hard to port to C++ because it is really built around Object Pascal features like a rich RTTI with explicitly exported properties, component streaming, "default" accessors, enums, sets, object instance method pointers and other features that C++ does not support at a language level. In C++ Builder the VCL is written in Object Pascal (and it even includes a Pascal compiler and linker), the language has been extended considerably to support Object Pascal-isms (e.g. __published section in classes, __property declarations, __fastcall calling convention that is Delphi-compatible, __delegate types for pointers to object instance methods, etc) and the runtime and library are written with some internal representation in mind (e.g. Pascal sets and strings are exposed to C++ using classes that have the same memory layout as the values the Pascal compiler creates). Of course some of those could be avoided if you do not care about a 1-to-1 port of FCL/LCL and compatibility with it, but still a lot of the functionality comes out of core Object Pascal features (since Borland had full control of the language, they designed both the language and API to complement each other, which is why it feels so weird in C++ Builder).

Beyond that, you'd really need an IDE with a visual designer like Lazarus as the entire API is designed to be manipulated in a visual way (e.g. while LCL does provide Qt/Gtk/Java/etc layout managers, they are really an afterthought in case you really want them and layout is mainly handled via anchors which have a dedicated visual editor - similarly the API uses a lot of custom collections that are meant to be edited with collection-specific editors in the IDE).

You could use Lazarus for that, but since Lazarus is meant to work with Free Pascal and does a lot of parsing (it essentially keeps the entire codebase, including all units and libraries as parsed nodes in memory) and automatic code editing (e.g. filling in the handler methods when you double click an event handler - or a component which fills the default event handler, navigating around the code, refactoring, removing empty handler methods, etc) you'll need to teach it enough C++ to be similar to writing a C++ compiler.

You could avoid the "porting to C++" part and using FCL/LCL directly from C++ in a similar way to what C++ Builder does... but then you'd need to convince some C++ compiler to include C++ Builder-like extensions and Free Pascal's developers to cooperate with said compiler's developers to ensure compatibility. You'd still need the Lazarus-needs-to-learn-C++ par though.

You could avoid all the above by making an "LCL inspired" (really, VCL inspired) library which vaguely resembles LCL for those who already know it but sticks with C++'s strengths. It wont be LCL/VCL though and wont take advantage of Lazarus, at which point you might as well make something that avoids the warts that LCL/Lazarus has.

Ah btw, did i mention the part where since LCL is essentially a cross-platform VCL reimplementation and VCL was designed only with Windows in mind, it has to implement a small subset of the Windows API for every platform and widgetset it supports? Yeah, are you sure you want to port that part too?

Honestly i'd love to see C++ Builder-like support in Lazarus and LCL, but practically i think it would be easier to make a new C++ Builder-like tool from scratch than make Lazarus, LCL, Clang (or GCC) and Free Pascal work together - even if we ignore the social/political side of convincing the developers behind those projects to work with the other projects.