[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

llvm-svn: 362650