Improve clang-cl discovery for Windows ARM64 builds

[justsmth]

Issues:

Addresses #1056

Description of changes:

On aarch64-pc-windows-msvc, the cc_builder requires clang-cl instead of MSVC's cl.exe — cl.exe can't handle .S ARM assembly files or several clang-style C flags we rely on. Previously, the build script only looked for clang-cl on PATH. If it wasn't there (e.g., building from a regular shell rather than a VS Developer Shell), the build would silently fall through to cl.exe and fail with a wall of inscrutable warnings and a linker error about missing .o files.

This change has the build script discover clang-cl inside the Visual Studio installations by leveraging the cc crate's existing VS toolchain discovery. If clang-cl isn't on PATH, we locate cl.exe through cc::windows_registry, walk up to the VS root, and probe for VC\Tools\Llvm\{ARM64,x64}\bin\clang-cl.exe. The result is cached via OnceLock to avoid redundant work across check_dependencies and build calls.

When clang-cl can't be found at all, the cc_builder now emits an actionable warning telling the user to install the "C++ Clang Compiler for Windows" VS component before the build fails.

Call-outs:

• The cmake_builder already handles this differently — it uses generator_toolset("ClangCL") which tells CMake's VS generator where to find clang-cl. The cmake_builder change here just moves the clang-cl dependency check out of the nasm-specific block so it applies independently.
• We intentionally don't fail the cc_builder's check_dependencies when clang-cl is missing. Doing so would cause get_builder to fall through to CmakeBuilder, which would produce a misleading "Missing dependency: cmake" error for users who don't have CMake installed.

Testing:

CI job (build-clang-cl-discovery-test) on windows-11-arm that removes clang-cl from PATH and verifies the build still succeeds via the fallback discovery path.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.65%. Comparing base (c358484) to head (670f444).
⚠️ Report is 366 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1060      +/-   ##
==========================================
- Coverage   95.80%   92.65%   -3.15%     
==========================================
  Files          61       71      +10     
  Lines        8143    10149    +2006     
  Branches        0    10149   +10149     
==========================================
+ Hits         7801     9404    +1603     
- Misses        342      452     +110     
- Partials        0      293     +293     

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