Upgrade LLVM 22 -> LLVM 23

[ShadovvBeast]

Summary

Upgrades the sig compiler from LLVM 22.1.6 to LLVM 23.1.1 across all build infrastructure.

Changes

• .github/workflows/build-llvm.yaml — Tag llvmorg-23.1.1, artifacts llvm-23-*, release tag llvm-23
• .github/workflows/release.yaml — Download from llvm-23 release, updated artifact names
• CMakeLists.txt — find_package(llvm/clang/lld 23)
• cmake/Findllvm.cmake — Version bounds 23 <= v < 24, search paths updated
• cmake/Findclang.cmake — Shared lib names .so.23, clang-cpp-23.0
• cmake/Findlld.cmake — Search paths and lib names updated to 23
• build.sig — Discovery step: "Find LLVM 23.x"
• src/zig_clang_driver.cpp — LLVM 23 C++ API adaptation
• src/zig_clang_cc1_main.cpp — LLVM 23 C++ API adaptation
• src/zig_clang_cc1as_main.cpp — LLVM 23 C++ API adaptation

Verified

• src/zig_llvm.h is UNCHANGED (external C interface preserved)
• All changes are within sig-owned file boundaries (sig-sync compatible)

Testing needed

• Run build-llvm.yaml workflow to produce LLVM 23 artifacts
• Run release.yaml to verify stage3 builds link correctly
• Run sig build test against LLVM 23 to verify no regressions
• Verify CI passes on all 3 platforms (x86_64-linux, aarch64-macos, x86_64-windows-gnu)