Allah forgives shirk
... Yet they [the Israelites] worshipped the calf even after Clear Signs had come to them; even so We forgave them; and gave Moses manifest proofs of authority. -- Sura 4:153
See also Sura 2:51-52, describing the same specific event. [This holds not only for the Israelites, Sura 6:76-78 asserts that even Abraham took the sun, the moon, and the stars as idols. This is sheer polytheism; yet Muslims respect Abraham as he is portrayed in the Qur'an as a great prophet, and nevertheless they believe that prophets are sinless.]
I suppose a Muslim could say that God abrogated his previous approach of granting forgiveness of idolatry but now does no longer forgive it. But is that so? In 6:88, after speaking of Abraham (verse 83), Isaac and Jacob (84), Moses, Job, Jesus, Joseph, Noah, David, Solomon, etc., Allah says, "Had they served other gods besides Him, their labours would have been vain indeed."
It seems obvious that since Abraham and Isaac are mentioned also (they preceeded Moses and the Hebrew slaves), and who were under the same command of "don't serve idols or your out of luck", that abrogation could not be possible in this case, since otherwise we would have to assume oscillating abrogations (no forgiveness, forgiveness, no forgiveness) suggesting that God cannot make up his mind. This would be utterly unworthy of God, who is eternally the same and as the All-wise does not need change his laws because he is surprised by new developments.
On the other hand, we can even find a general statement about the forgiveness of shirk, since Sura 25:68-71 speaks about people who worship gods other than God and when they repent, believe, and work righteousness then God not only forgives them but He even substitutes their sins (sayyi'aat) with righteouss credit (