Expression will be checked against expected type later.
Theoretically, this is not very good, but it aligns with the old
inference, plus it helps avoiding multiple type mismatch diagnostics.
rewrite type inference for 'when' using special constructs.
This fixes several type inference issues for 'when':
KT-9929, KT-9972, KT-10439, KT-10463
along with some other diagnostics-related issues.
Mikhail Zarechenskiy