Custom expression: improve the type-checker (#19187)

Use an improved semantic validation layered on top of the existing
resolver (around ~40 LoC) instead of its own dedicated logic
(~240 LoC).  Not only the improved type-checker is leaner and faster,
it's also more accurate in some corner cases.
The type-checking process is also deferred to the compilation stage,
instead of earlier in the parsing stage.