Add CanBeConcatenated to ArgInfo::cmp

[2over12]

This is an approach to fixing #2505 by adding CanBeConcatenated to ArgInfo::cmp, allowing -fplugin=x to match against CanBeConcatenated. Also adds a test case for fplugin itself

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.35%. Comparing base (9de8d18) to head (6e423c5).

Files with missing lines	Patch %	Lines
src/compiler/args.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2506      +/-   ##
==========================================
+ Coverage   70.34%   70.35%   +0.01%     
==========================================
  Files          64       64              
  Lines       35318    35337      +19     
==========================================
+ Hits        24845    24862      +17     
- Misses      10473    10475       +2     

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