Extensions: diagnostic quality follow-ups

[jcouv]

This issue is referenced in source.
It tracks scenarios that were identified as needing more work on diagnostic quality.

• Consider removing or adjusting the reported argument position for WRN_BadArgRef
• Some diagnostics refer to extension blocks as "types"
• Report what operator methods conflict Extensions: improve diagnostics for operators #80928
• Some diagnostics report extension blocks without a containing type (extension(int) instead of E.extension(int)) Extensions: address some diagnostic quality issues #80827
• Diagnostics for method vs. property or property vs. property conflicts are uninformative Extensions: address some diagnostic quality issues #80827

Relates to test plan #76130