Rollup merge of #24207 - kmcallister:llvm-plugin, r=brson

 r? @brson

I'm using this to integrate rustc with [american-fuzzy-lop](http://lcamtuf.coredump.cx/afl/). Building with afl instrumentation is no different from loading any other plugin library.

I'd like this PR to include a `run-make` test with a custom LLVM pass; however I'm not sure it's worth the trouble of building C++ code and linking LLVM from the test suite (are there existing tests that do this?)