Introduce Unliftable for Quasiquotes

Resubmission of https://github.com/scala/scala/pull/3118 with additional bugfixes for SI-8008,  SI-8016, SI-8009, SI-6842, SI-7979, SI-7980, SI-7996, SI-7789 and SI-7154. Best viewed per-commit. 

review by @retronym @xeno-by