add autofix comments + diagnostic on discards

[Techatrix]

This idea has been brought up in #1623.

Here is what I've changed:

• add a // autofix comment after every discard that is added by autofix
• only removed unnecessary discard if they have the autofix comment
• show a diagnostic on every discard that has the autofix comment

Suggestions on a better better diagnostic message or autofix comment are welcome.

Here is what I get shown when autofix adds a discard to my code:

Note that I am using the usernamehw.errorlens extension that shows diagnostics messages inside the document.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d1ad449) 75.55% compared to head (f144116) 75.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1665      +/-   ##
==========================================
+ Coverage   75.55%   75.64%   +0.08%     
==========================================
  Files          33       33              
  Lines        9067     9074       +7     
==========================================
+ Hits         6851     6864      +13     
+ Misses       2216     2210       -6     

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