[build images] Upgrade LLVM from 19 to 21

Update all LLVM version references from 19.1.7 to 21.1.0 across:
- CMake configuration (SetupLLVM.cmake)
- CI/CD (Dockerfile, GitHub Actions workflows, CLAUDE.md)
- Bootstrap scripts (bash and PowerShell)
- Nix configurations (flake.nix, shell.nix)
- Documentation (CONTRIBUTING.md, contributing.mdx, building-windows.mdx)
- Build scripts (build-jsc.ts, run-clang-format.sh)
- Test harness (node-napi-tests/harness.ts)
- Crash handler (llvm-symbolizer fallback)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/napi/node-napi-tests/harness.ts

@@ -20,8 +20,8 @@ export async function build(dir: string) {
       // so we make it use clang instead
       ...(process.platform == "linux" && isCI
         ? {
-            CC: !isMusl ? "/usr/lib/llvm-19/bin/clang" : "/usr/lib/llvm19/bin/clang",
-            CXX: !isMusl ? "/usr/lib/llvm-19/bin/clang++" : "/usr/lib/llvm19/bin/clang++",
+            CC: !isMusl ? "/usr/lib/llvm-21/bin/clang" : "/usr/lib/llvm21/bin/clang",
+            CXX: !isMusl ? "/usr/lib/llvm-21/bin/clang++" : "/usr/lib/llvm21/bin/clang++",
           }
         : {}),
     },