C front-end: tolerate type differences with asm renaming

[tautschnig]

The example found in glibc shows how asm renaming may be combined with changes in return type names (even when there isn't actually a bit-level difference amongst the types). Weakening the asm-renamed declaration by marking it "incomplete" works around this problem.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.39%. Comparing base (22d25d1) to head (f6fd739).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7584      +/-   ##
===========================================
- Coverage    80.39%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207403   207405       +2     
  Branches        73       73              
===========================================
- Hits        166752   166751       -1     
- Misses       40651    40654       +3     

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