Everywhere where EU4 and C++ programming and compiling UE projects is mentioned it is said about Visual Studio.
As far as I understand UE projects are meant to be worked with in Visual Studio (in the part that requires compiler).
As a person who dislikes Visual Studio and is used to Borland/Embarcadero compilers I wish to ask:
– are there any official instructions on how to compile UE projects (and source code) with RAD Studio?
– are there any examples of people successfully working with UE code in RAD Studio/Embarcadero C++ Builder?
– is UE even meant to be compiled with compilers other than Visual Studio?
Does anyone even use RAD Studio here?
No, it’s not possible. UE has a complete build system that is custom tailored to support specific platforms and compilers, so that alone would make it impossible since there is no Embarcadero toolchain in UnrealBuildTool. Though if you REALLY wanted to you could probably make one. But then you’re going to be up against trying to compile a codebase of 10K source files using an unsupported compiler (and Embarcadero’s compiler wasn’t very up to date with language standards last time I used it, though that was some time ago). Even the Intel compiler (which is extremely similar to MSVC in many ways and can often be used as a drop-in replacement) can’t compile the engine without some significant patching. So basically, it can’t be done.
As for your third question, yes, there is Clang 
Clang is supported on both Windows and Linux.
Wrong reply: I’m assume you’ll be able to compile it (but not totally sure) since there are versions for Mac and Linux without Visual Studio
Edit: I assume that the previous reply written meanwhile I was writtening seems to be the right one.