traditionally, this required a linker that understands there is ir in the files. in practice, i don't believe this has been a problem for many years now (and again, was only a problem in the open source world, so saying it's related to the language is kind of strange.).

Every good production C++ compiler has had some form of link time optimization for many years.

IBM's, for example, has been happily cross-optimizing between C++, java, fortran, PL/IX, etc without any issues, going on at least 15, maybe 25+ years now (I know it's 15 for sure, i suspect it's closer to 25).