Symbols for which canBeOmittedFromSymbolTable() is true should be
treated as private externs. This diff tries to do that by unsetting the
ExportDynamic bit. It seems to mostly work with the FullLTO backend, but
with the ThinLTO backend, the local_unnamed_addr symbols still fail to
be properly hidden. Nonetheless, this is a step in the right direction.
I've documented all the remaining differences between our behavior and
LD64's in the lto-internalized-unnamed-addr.ll test.