[libc++] Add a merge driver that can apply clang-format (#73712)

In preparation for the moment when we'll clang-format the whole code
base, this patch adds a script that can be used to rebase patches across
clang-format changes mechanically, without requiring manual
intervention.

See https://discourse.llvm.org/t/rfc-clang-formatting-all-of-libc-once-and-for-all.