Add tests for LLVM symbol conflicts

[brson]

re https://github.com/brson/wasm-opt-rs/issues/154

We are fixing this issue with LLVM namespace conflicts by adding an optional "dwarf" feature, and testing in both configurations, but don't currently have a place to put a test of the underlying bug - that compiling with "dwarf" induces linker errors when also linking to llvm.

We'll need to dedicate a crate to this problem - link to both llvm and wasm-opt, with an optional dwarf feature; write a test case ... somewhere, that compiles in both configurations and confirms one fails and one succeeds.