[UTC] Fix SyntaxWarning on Python 3.12 (#82327)

On Python 3.12 we now get a warning in common.py:

llvm/utils/UpdateTestChecks/common.py:488: SyntaxWarning: invalid escape
sequence '\s'

This fixes it by using a raw string literal, see
https://github.com/llvm/llvm-project/pull/78036 and
https://docs.python.org/3/library/re.html