Improve build scripts for bash compatibility and update README

[Septa2112]

Issue

Now running ./build_llvm.sh will cause an issue: ./build_llvm.sh: 17: source: not found.

Because the script is executed with the system default shell (sh), which does not support the source command used in the script, it will cause the Python virtual environment not to activate actually.

How to Fix

The script should either be explicitly run using bash (e.g., bash ./build_llvm.sh) or the script itself should specify bash as the default shell by adding the shebang line #!/usr/bin/env bash at the top.

My improvements:

• Updated the README.
• Modified the shebang line.
• Added set -e to make the script exit immediately if any command fails.