wasm: support source maps for the unknown target

[est31]

Source maps are the analogon to debuginfo in the js world, including wasm. So we should generate them.

Emscripten apparently already is able to emit them, at least this example contains source maps: https://yurydelendik.github.io/old-man-sandbox/rust-wasm-hey/hey.html Maybe it's not upstreamed yet, idk.

I guess the bulk of this work would reside inside llvm, but I'm still filing a bug here to track support.

[brandf]

Is there any update, or workaround on this? Rust+wasm is compelling for my scenarios, but without source maps, it's not a viable option.

[est31]

@brandf use the emscripten based backend. Other than that, no idea.

[est31]

Hrmmm did #47883 fix this? Need to check...