[WebAssembly] Improve lto/comdat.ll test. NFC.
We were not previously testing the comdat exclusion in bitcode objects
because we were linking two copies of the .bc file and the
linkonce_odr linkage type was removing the duplicate _start at
the LTO stage.
Now we link an bitcode and non-bitcode version both of which contains a
copy of _start. We link them in both orders, which means this test will
fail if comdat exclusion is not working correctly in bitcode parsing.
Differential Revision: https://reviews.llvm.org/D62923