Disable preprocessor cache with -MD and friends

[glandium]

Also hook up too_hard_for_preprocessor_cache_mode, because it wasn't.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 42.55%. Comparing base (0cc0c62) to head (df191d6).
Report is 81 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/c.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2195       +/-   ##
===========================================
+ Coverage   30.91%   42.55%   +11.64%     
===========================================
  Files          53       53               
  Lines       20112    20328      +216     
  Branches     9755     9684       -71     
===========================================
+ Hits         6217     8650     +2433     
- Misses       7922     7976       +54     
+ Partials     5973     3702     -2271     

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[sylvestre]

could you please add a log entry here ? might be hard to debug otherwise for people who might want to use it

[glandium]

That would need to mention what specifically makes it too hard for the preprocessor cache, though, otherwise the log is not going to be useful (like cannot_cache!). Maybe more of a followup?

[glandium]

Done in #2197. I'll rebase this one when that one is merged.