Auto convert reference type in return type as unit type

[fzyzcjy]

Changes

Please list issues fixed by this PR here, using format "Fixes #the-issue-number".

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

Attention: Patch coverage is 80.95238% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 97.85%. Comparing base (328ac0e) to head (f02b8df).
Report is 7 commits behind head on master.

❗ Current head f02b8df differs from pull request most recent head 6825347

Please upload reports for the commit 6825347 to get more accurate results.

Files	Patch %	Lines
...brary/codegen/parser/mir/function_parser/output.rs	80.95%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2017      +/-   ##
==========================================
- Coverage   97.88%   97.85%   -0.03%     
==========================================
  Files         414      414              
  Lines       16847    16865      +18     
==========================================
+ Hits        16490    16504      +14     
- Misses        357      361       +4     

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