There's a merged pull request on the void-packages repository

ldc and dependents update to allow LLVM7 removal
https://github.com/void-linux/void-packages/pull/20841

Description:
This PR updates `ldc` and its dependents, which were the last remaining dependents of `llvm7`. As a consequence, I replaced `llvm7` and `libllvm7` with a metapackage in the same fashion as `llvm8` and `llvm6.0`.

Because `ldc` and `gtkd` set `nocross`, I can only test this on x86_64.