rustc: Update LLVM, remove dead wasm code

This commit updates the LLVM branch to the rebased version of the
upstream release/8.x branch. This includes a wasm patch which means that
the `rewrite_imports` pass in rustc is no longer needed (yay!) and we
can instead rely on `wasm-import-module`, an attribute we're already
emitting, to take care of all the work.

src/librustc_codegen_llvm/back/link.rs

@@ -698,7 +698,6 @@ fn link_natively(sess: &Session,
     }
 
     if sess.opts.target_triple.triple() == "wasm32-unknown-unknown" {
-        wasm::rewrite_imports(&out_filename, &codegen_results.crate_info.wasm_imports);
         wasm::add_producer_section(
             &out_filename,
             &sess.edition().to_string(),