debug: handle storage-free Python attributes

[cpunion]

Problem

Python module attributes are frontend pseudo-variables. LLGo loads and stores them through Python C API calls, so their ssa.Expr has a source-facing type but no native LLVM global or address.

With DWARF enabled, the generic global path tried to derive a native debug type and attach a DIGlobalVariableExpression to that absent storage. Compiling _testpy/list therefore reached pyVarTy.Underlying and panicked with don't call.

Changes

• make ssa.Builder.DIGlobal ignore expressions that have no native LLVM storage
• keep ordinary Go and C globals on the existing DWARF path
• do not emit a fake address or misleading DW_TAG_variable for dynamically resolved Python attributes
• add a focused unit test using a real storage-free Python variable expression
• run the complete _testpy fixture set with DWARF enabled on supported hosts

Fixes #2117.

Verification

macOS arm64, Go 1.26.5 / LLVM 19:

• all 8 _testpy fixtures pass with DWARF
• full go test ./ssa passes
• standards-based DWARF matrix passes at O0, O1, O2, O3, Os, and Oz
• the changed DIGlobal paths are covered directly

Ubuntu arm64 container, limited to 2 CPUs, 15 GiB memory, and 512 PIDs:

• 7 stable _testpy fixtures pass with DWARF
• standards-based DWARF matrix passes at all six optimization levels
• _testpy/matrix still reproduces the same existing runtime crash with and without DWARF; this PR does not add a skip or hide that baseline failure

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.