C front-end: fallthrough and __fallthrough__ are the same attribute

[tautschnig]

We already supported the former, but were missing the latter form.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5730 (f215721) into develop (111b51e) will decrease coverage by 0.64%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5730      +/-   ##
===========================================
- Coverage    73.55%   72.90%   -0.65%     
===========================================
  Files         1431     1425       -6     
  Lines       155248   154279     -969     
===========================================
- Hits        114189   112482    -1707     
- Misses       41059    41797     +738     

Impacted Files	Coverage Δ
src/ansi-c/parser.y	77.83% <100.00%> (+<0.01%)	⬆️
src/ansi-c/scanner.l	61.71% <100.00%> (+0.10%)	⬆️
src/goto-instrument/accelerate/accelerate.h	0.00% <0.00%> (-100.00%)	⬇️
src/goto-instrument/accelerate/path_enumerator.h	0.00% <0.00%> (-100.00%)	⬇️
...c/goto-instrument/accelerate/sat_path_enumerator.h	0.00% <0.00%> (-100.00%)	⬇️
...goto-instrument/accelerate/overflow_instrumenter.h	0.00% <0.00%> (-100.00%)	⬇️
...oto-instrument/accelerate/polynomial_accelerator.h	0.00% <0.00%> (-100.00%)	⬇️
...trument/accelerate/enumerating_loop_acceleration.h	0.00% <0.00%> (-100.00%)	⬇️
src/goto-instrument/accelerate/scratch_program.h	0.00% <0.00%> (-92.86%)	⬇️
...rc/goto-instrument/accelerate/acceleration_utils.h	0.00% <0.00%> (-75.00%)	⬇️
... and 191 more

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[martin-cs]

Fine although I guess us ignoring it means that we are saying this is too syntactic for us. As there is a -W flag for it, I thing that is reasonable.