[clang-tidy] Install run-clang-tidy.py in bin/ as run-clang-tidy

The run-clang-tidy.py helper script is supposed to be used by the
user, hence it should be placed in the user's PATH. Some
distributions, like Gentoo [1], won't have it in PATH unless it is
installed in bin/.

Furthermore, installed scripts in PATH usually do not carry a filename
extension, since there is no need to know that this is a Python
script. For example Debian and Ubuntu already install this script as
'run-clang-tidy' [2] and hence build systems like Meson also look for
this name first [3]. Hence we install run-clang-tidy.py as
run-clang-tidy, as suggested by Sylvestre Ledru [4].

1: https://bugs.gentoo.org/753380
2: https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/blob/60aefb14171ab5c3867a0081844b507fc9f6e015/debian/clang-tidy-X.Y.links.in#L2
3: https://github.com/mesonbuild/meson/blob/b6dc4d5e5c6e838de0b52e62d982ba2547eb366d/mesonbuild/scripts/clangtidy.py#L44
4: https://reviews.llvm.org/D90972#2380640

Reviewed By: sylvestre.ledru, JonasToth

Differential Revision: https://reviews.llvm.org/D90972