goto-cc: fix dead -fsingle-precision-constant query

[tautschnig]

goto_cc_cmdlinet stores long options with a single leading '-' stripped, so the existing isset("-fsingle-precision-constant") (with the dash) never matched and the option was silently dead. Query it as "fsingle-precision-constant", consistent with the other -f options.

Add regression/goto-gcc/fsingle_precision_constant, which compiles with -fsingle-precision-constant and checks via _Static_assert that an unsuffixed floating-point constant is float rather than double. Before this fix the flag was ignored, the constant stayed double, and the assertion (hence conversion) failed -- so the test fails without the fix.

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[Copilot]

Pull request overview

Note

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Adds support for MS-style extensions needed to correctly typecheck and lay out tagged anonymous struct/union members (e.g., Linux kernel patterns) when compiling in GCC/Clang mode with -fms-extensions.

Changes:

• Introduce a new ansi_c.ms_extensions config flag (default false) and reset it in configt::set.
• Enable the flag when -fms-extensions is passed through goto-gcc.
• Extend compound member typechecking to accept tagged anonymous struct/union members only when ms_extensions is enabled, and add a regression test validating layout + member injection.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/util/config.h	Adds ansi_c.ms_extensions configuration flag.
src/util/config.cpp	Initializes/resets ansi_c.ms_extensions during configt::set.
src/goto-cc/gcc_mode.cpp	Parses -fms-extensions and maps it to config.ansi_c.ms_extensions.
src/ansi-c/c_typecheck_type.cpp	Implements conditional acceptance of tagged anonymous struct/union members.
regression/goto-gcc/ms_extensions_anonymous_member/test.desc	Adds a regression test driver for the new behavior.
regression/goto-gcc/ms_extensions_anonymous_member/main.c	Adds a regression C file asserting layout and injected member access.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.72%. Comparing base (17ab9f4) to head (51e6e0c).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@ Coverage Diff @@## develop #9050 +/- ##
========================================
Coverage 80.72% 80.72% ========================================
Files 1714 1714 Lines 189915 189915 Branches 73 73 ========================================
+ Hits 153311 153313 +2 + Misses 36604 36602 -2 

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[tautschnig]

We'll have to consider whether to take this one or #9022. Edit: picked #9022 with slight updates.