Error if we see an alias to a declaration.

In ELF and COFF an alias is just another offset in a section. There is no way
to represent an alias to something in another file.

In MachO, the spec has the N_INDR type which should allow for exactly that, but
is not currently implemented. Given that it is specified but not implemented,
we error in codegen to avoid miscompiling but don't reject aliases to
declarations in the verifier to leave the option open of implementing it.

In the past we have used alias to declarations as a way of implementing
weakref, which is why it exists in some old tests which this patch updates.

llvm-svn: 194705