[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>

scripts/lldb-inline-tool.cpp

@@ -20,8 +20,8 @@
  * On Linux:
  *   c++ -std=c++17 -o lldb-inline lldb-inline-tool.cpp \
  *       -llldb \
- *       -L/usr/lib/llvm-14/lib \
- *       -I/usr/lib/llvm-14/include
+ *       -L/usr/lib/llvm-21/lib \
+ *       -I/usr/lib/llvm-21/include
  * 
  * USAGE:
  * ------