After upgrading our (ClickHouse's) libcxx from 15 to 16, the compiler
started to complain about usage of an incomplete type "RoseInstruction"
in this (header) function:

  void RoseProgram::replace(Iter it, std::unique_ptr<RoseInstruction> ri) {
    ...

The reason is that libcxx 16 is the first version which implements C++23
constexpr std::unique_ptr (P2273R3, see (*)). RoseProgram::replace()
happens to be be const-evaluatable and the compiler tries to run
std::unique_ptr's ctor + dtor. This fails because at this point
RoseInstruction isn't defined yet.

There are two ways of fixing this:
1. Include rose_build_instruction.h (which contains RoseInstruction)
   into rose_build_program.h. Disadvantage: The new include will
   propagate transitively into all callers.
2. Move the function implementation into the source file which sees
   RoseInstruction's definition already. Disadvantage: Template
   instantiation is no longer automatic, instead there must be either a)
   explicit template instantiation (e.g. in rose_build_program.cpp) or
   b) all callers which instantiate the function must live in the same
   source file and do the instantiations by themselves. Fortunately, the
   latter is the case here, but potential future code outside
   rose_build_program.cpp will require ugly explicit instantiation.

(*) https://en.cppreference.com/w/cpp/23