Add idempotency check to LLVM installation script

[Copilot]

Fix LLVM being downloaded every time bootstrap runs

• Understand the problem: install-llvm.sh always downloads LLVM without checking if it exists
• Identify the solution: Add check in install-llvm.sh to skip download if LLVM is already installed
• Modify install-llvm.sh to check if LLVM is already installed before downloading
• Test the fix by running the script multiple times
• Verify LLVM download is skipped when already installed
• Address code review feedback to check all three tools
• Run security checks (N/A for bash scripts)
• Eliminate code duplication by extracting symlink creation to function

Original prompt

/task I found llvm is always downloaded everytime I run bootstrap, please help me fix it

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[zesonzhang]

@copilot these lines are duplicated:
"""
echo "Creating symlinks in /usr/local/bin..."
sudo ln -sf "${INSTALL_DIR}/bin/clang-format" "/usr/local/bin/clang-format-19"
sudo ln -sf "${INSTALL_DIR}/bin/clang-tidy" "/usr/local/bin/clang-tidy-19"
sudo ln -sf "${INSTALL_DIR}/bin/clang-apply-replacements" "/usr/local/bin/clang-apply-replacements-19"
"""

[Copilot]

@copilot these lines are duplicated:
"""
echo "Creating symlinks in /usr/local/bin..."...

Fixed in commit 8e5a120. Extracted the symlink creation code into a create_symlinks() function to eliminate the duplication.