[WebAssembly] Cleanup WasmObjectWriter.cpp. NFC

- Use auto where appropriate
- Use early return to reduce nesting
- Remove stray comment line
- Use C++ foreach over explicit iterator

Differential Revision: https://reviews.llvm.org/D34477

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305971 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed