wasm: Pass `--no-demangle` to LLD

Our mangling scheme is not C++'s, so tell LLD to not demangle anything
so we can handle Rust-specific demangling ourselves.

src/librustc_codegen_ssa/back/linker.rs

@@ -1037,6 +1037,11 @@ impl<'a> Linker for WasmLd<'a> {
         // indicative of bugs, let's prevent them.
         self.cmd.arg("--fatal-warnings");
 
+        // LLD only implements C++-like demangling, which doesn't match our own
+        // mangling scheme. Tell LLD to not demangle anything and leave it up to
+        // us to demangle these symbols later.
+        self.cmd.arg("--no-demangle");
+
         ::std::mem::replace(&mut self.cmd, Command::new(""))
     }