Revert "[WebAssembly] Refactor and fix emission of external IR global decls"

This reverts commit 00bf4755.

This change broke the emscripten builder (among other things):

https://ci.chromium.org/ui/p/emscripten-releases/builders/try/linux/b8823500584349280721/overview

Sample failure:

```
test_unistd_unlink (test_core.core0) ...
wasm-ld: error: symbol type mismatch: __stdio_write
>>> defined as WASM_SYMBOL_TYPE_FUNCTION in /usr/local/google/home/sbc/dev/wasm/emscripten/cache/sysroot/lib/wasm32-emscripten/libc-debug.a(__stdio_write.o)
>>> defined as WASM_SYMBOL_TYPE_DATA in /usr/local/google/home/sbc/dev/wasm/emscripten/cache/sysroot/lib/wasm32-emscripten/libc-debug.a(stderr.o)
```