This is an example implementation for adding .eh_frame pieces to the map file. See the RFC on the mailing list: http://lists.llvm.org/pipermail/llvm-dev/2018-February/120993.html. Credit to @andrewng for this.
I wouldn't expect to check this in as-is. It's intended only as a demonstration of how this might be achieved. I imagine a similar approach would work for meregeable section pieces.