Improve eager type resolution error message

Improve eager type resolution error message

This PR improves the span of eager resolution type errors referring to indexing and field access to use the base span rather than the whole expression.

Also a note "Type must be known at this point." is added to where we at some point in the past emitted the "type must be known at this context" error, so that early failures can be differentiated and will hopefully be less surprising.

Fixes #50692 (or at least does the best we can for the moment)

r? @estebank