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 71.06%. Comparing base (1332701) to head (d605d55).
⚠️ Report is 8 commits behind head on main.

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2506   +/-   ##
=======================================
  Coverage   71.05%   71.06%           
=======================================
  Files          64       64           
  Lines       35359    35378   +19     
=======================================
+ Hits        25124    25140   +16     
- Misses      10235    10238    +3     

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