[clang] [Clang] Add 'CLANG_ALLOW_IMPLICIT_RPATH' to enable toolchain use of -rpath (PR #82004)

[Tom Stellard via cfe-commits]

tstellar wrote:

> > > I believe right now the `rtlib-rpath` points to the path `${CLANG_BINARY}../lib/${HOST_TRIPLE}/`. I think it's definitely reasonable to not put this on system libraries if that's a solution, since we can generally assume it's a global installation and already covered by the user's library path. Does the Fedora policy only disallow `rpath` on system installs then?
> > 
> > 
> > Right, any official Fedora binaries we build can't have rpath in them, but users can still use rpath in their own applications if they want.
> 
> I'd be okay with that solution. The majority of the issues come from people on cluster machines with several conflicting versions of the libraries. The only question is how to determine what a system library is. We could possible just hardcode a few paths depending on the host triple's operating system.

I'm not a huge fan of trying to auto-detect settings based on the host OS.  What if we just added an option like --exclude-rpath-dir= which would let a user or a distro specify a list of directories that aren't allowed to be added as rpath.  Now that we have config file support it would be easy for distros to add a config file with the list of its own system directories.

https://github.com/llvm/llvm-project/pull/82004