resolve: improve "try using the enum's variant"
Fixes#73427.
This PR improves the "try using the enum's variant" suggestion:
- Variants in suggestions would not result in more errors (e.g. use of a struct variant is only suggested if the suggestion can trivially construct that variant). Therefore, suggestions are only emitted for variants that have no fields (since the suggestion can't know what value fields would have).
- Suggestions include the syntax for constructing the variant. If a struct or tuple variant is suggested, then it is constructed in the suggestion - unless in pattern-matching or when arguments are already provided.
- A help message is added which mentions the variants which are no longer suggested.
All of the diagnostic logic introduced by this PR is separated from the normal code path for a successful compilation.
r? `@estebank`
deec530523