LLVM will transform your mutable program into immutable one anyway because otherwise it's much harder if not impossible to write a lot of optimizations or validations.
You can collapse a lot of the additional copies at compile time even for C
That's not really the same thing though - that's a choice of representation for optimization purposes. LLVM won't turn a pointer access into a value copy - it can't, because that would change program behavior.
The distinction here is that in a lot of cases, you really do want to edit data in one location and there's no good reason for it to be copied except that it makes the program easier to reason about (exceptions apply for when things like cache locality considerations come into play).
I seem to recall that in many cases it is in fact permitted for C compiler (probably C++ as well) to turn pointer access into value copy so long as the rather flexible semantics are still kept.
The main saving grace is that usually you're only dealing with register copies...
You can collapse a lot of the additional copies at compile time even for C