Error instead of crash when generating trees referring to skolems

Previously, the valueOf inline call succeeded (because the ValueOf synthesizer
calls `tpd.ref` which calls `tpd.singleton`), leading to an invalid tree which
crashed in the backend with: "assertion failed: Cannot create ClassBType from
NoSymbol".

Fixed by throwing a TypeError from `tpd.singleton`, unfortunatley this means
tests/neg/i8623.scala gets a worse error message because the implicit search
fails early, but arguably this is more correct.

[Cherry-picked 6e4b0a0][modified]

Author: tgodzik