Make module std; and module std.compat; work with Clang

[cpplearner]

This adds Clang diagnostic suppressions as requested in #5975, and makes the module-related tests work with clang-cl.

Thanks to LLVM-98761 and LLVM-89772, clang-cl understands the Clang-style options needed to produce and consume Built Module Interfaces (BMIs). This is good enough for our test purposes.

I need to guard a line that calls source_location::column, because Clang and MSVC produce different values. This isn't specific to modules. The difference can also be found in the P1208R6_source_location test.

I also need to workaround a bug, where clang thinks the declaration of _alloca in <malloc.h> has conflicting ownership. This looks similar to LLVM-37969, though the trigger is different. I've added a repro to that issue. Note that the bug only affects ARM64/ARM64EC, because <intrin.h> is included in the global module fragment, which includes <malloc.h> on x64/x86.

Fixes #5975

[Copilot]

Pull request overview

This PR improves support for building and testing the STL named modules (module std; and module std.compat;) with Clang/clang-cl, primarily by adding Clang-specific diagnostic suppressions and adjusting module-related test build steps to use Clang’s module BMI precompile flow.

Changes:

• Add Clang diagnostic push/pop suppressions in std.ixx and std.compat.ixx (and a Clang/ARM64EC workaround include in std.ixx).
• Extend the modules test matrix to include clang-cl configurations and refine the per-config flag composition.
• Update module-related custom build scripts (Perl + Lit Python formats) to add Clang-specific --precompile steps and module compilation flags.

Show a summary per file

File	Description
tests/std/tests/modules_20_matrix.lst	Adds clang-cl rows and restructures module test matrix flags.
tests/std/tests/VSO_1775715_user_defined_modules/custombuild.pl	Adds a Clang branch that precompiles the module interface before building.
tests/std/tests/VSO_1775715_user_defined_modules/custom_format.py	Adds Clang-specific Lit build steps for precompiling module BMIs.
tests/std/tests/P2465R3_standard_library_modules/custombuild.pl	Adds a Clang branch to precompile std.ixx/std.compat.ixx before building.
tests/std/tests/P2465R3_standard_library_modules/custom_format.py	Adds Clang-specific Lit build steps for precompiling standard library module BMIs.
tests/std/include/test_header_units_and_modules.hpp	Adjusts tests for Clang/ARM64EC generator workaround and Clang source_location::column() differences.
stl/modules/std.ixx	Adds Clang diagnostic suppressions and a Clang-specific include workaround in the global module fragment.
stl/modules/std.compat.ixx	Adds a global module fragment and Clang diagnostic suppression for reserved module identifier warnings.

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• Files reviewed: 8/8 changed files
• Comments generated: 4

[cpplearner]

I'll keep this PR as draft until the next toolset update brings Clang 22.

Good news: Clang 22 fixed LLVM-173689, which makes the -fmodule-output option working. This will simplify the scripts.

Bad news: Clang 22 introduced -WTU-local-entity-exposure which brings a new issue, LLVM-195661, that needs workaround. (Not a showstopper: the issue only affects <complex> and an existing fallback path can be used.)

[StephanTLavavej]

Thank you; I will try to prioritize this when I can focus on the STL again. (I believe I'll need to write Perl to make this work in the internal test harness, like most things that need custom Python for the GitHub test harness, which makes this way more obnoxious to deal with, through no fault of your own.)

[cpplearner]

The -fmodule-output approach will minimize the custom logic. All we need to do is telling Clang that the ixx files are modules.

It will basically look like:

diff --git a/tests/std/tests/P2465R3_standard_library_modules/custom_format.py b/tests/std/tests/P2465R3_standard_library_modules/custom_format.py
index d7d6a69f..1cca3ff6 100644
--- a/tests/std/tests/P2465R3_standard_library_modules/custom_format.py
+++ b/tests/std/tests/P2465R3_standard_library_modules/custom_format.py
@@ -22,7 +22,12 @@ class CustomTestFormat(STLTestFormat):
         classicCpp = os.path.join(sourceDir, 'classic.cpp')

         # Dependency order is important here:
-        inputPaths = [stdIxx, stdCompatIxx, testCpp, test2Cpp, test3Cpp, test4Cpp, classicCpp]
+        moduleUnits = [stdIxx, stdCompatIxx]
+        traditionalUnits = [testCpp, test2Cpp, test3Cpp, test4Cpp, classicCpp]
+        if 'clang' in test.config.available_features:
+            inputPaths = ['-x', 'c++-module', *moduleUnits, '-x', 'none', *traditionalUnits]
+        else:
+            inputPaths = [*moduleUnits, *traditionalUnits]

         cmd = [test.cxx, *inputPaths, *test.flags, *test.compileFlags]

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