run-clang-tools: run multiprocessing.Pool as context manager

run-clang-tools: run multiprocessing.Pool as context manager

`multiprocessing.pool.Pool()` should be used as a context manager so
Python can free its internal resources and do a proper cleanup.[1]

While at it move the code to read the `compiler_commands.json` so the
opened file can be closed before the sub-processes are fork()ed.

Link: https://docs.python.org/3/library/multiprocessing.html#multiprocessing.pool.Pool
Signed-off-by: Philipp Hahn <phahn-oss@avm.de>
Link: https://patch.msgid.link/40180613bef84946c45d6fbeb4bb274573cd0beb.1778849135.git.phahn-oss@avm.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>