C front-end: fix processing of alignment and packing attributes

[tautschnig]

Packing needs to be evaluated as part of the definition of the type while alignment applies also when using a type. The position of the attribute also requires extra care as some positions are accepted by GCC (and warned about by Clang) while actually being ignored.

Fixes: #8443

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[codecov]

Codecov Report

❌ Patch coverage is 92.68293% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.02%. Comparing base (bacf91c) to head (200002c).
⚠️ Report is 54 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/ansi-c/c_typecheck_type.cpp	91.30%	2 Missing ⚠️
src/ansi-c/parser.y	94.44%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8454      +/-   ##
===========================================
+ Coverage    80.01%   80.02%   +0.01%     
===========================================
  Files         1700     1700              
  Lines       188344   188382      +38     
  Branches        73       78       +5     
===========================================
+ Hits        150701   150754      +53     
+ Misses       37643    37628      -15     

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[kroening]

Is that a requirement in the standard?

[tautschnig]

C11 6.2.8 "Alignment of objects" includes the following sentence: "Every valid alignment value shall be a nonnegative integral power of two."

[Copilot]

Pull request overview

Fixes C front-end handling of GCC packed and aligned attributes so packing is applied during type definition while alignment also affects later uses, and ignores attribute placements that GCC accepts but ignores.

Changes:

• Update grammar action for sue_declaration_specifier to drop ignored packed/aligned attributes before merging specifiers.
• Add alignment validation (power-of-two) and introduce align_to to combine alignments across redefinitions / typedef uses.
• Add a regression test covering many attribute placements and interactions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
src/ansi-c/parser.y	Filters out ignored packed/aligned attributes in a specific specifier context to match GCC behavior.
src/ansi-c/c_typecheck_type.cpp	Validates alignment values and combines alignments when reusing/redefining types.
regression/ansi-c/gcc_attributes16/test.desc	Adds expected-output configuration for the new regression test.
regression/ansi-c/gcc_attributes16/main.c	Adds C source covering many packed/aligned placements and expected sizeof results.

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