We were not previously testing the comdat exclusion with bitcode
object parsing because we were linking two copied of the .bc file and
the linkonce_odr linkage type was removing the duplicate _start
function during LTO.
Now we link an bitcode and non-bitcode version both of which contains
a copy of _start. And we link them in both orders, which means this
test will fail if comdat exclusion is not working correctly in the
parsing of the bitcode file.