New fix for pr17535.

This is a fixed version of r193161. In order to handle

    void foo() __attribute__((alias("bar")));
    void bar() {}
    void zed() __attribute__((alias("foo")));

it is not enough to delay aliases to the end of the TU, we have to do two
passes over them to find if they are defined or not.

This can be implemented by producing alias as we go and just doing the second
pass at the end. This has the advantage that other parts of clang that were
expecting alias to be processed in order don't have to be changed.

This patch also handles cyclic aliases.