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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > We could make this logic more complicated by checking the system automatically `execute_command(lsb_release)` or something.
> 
> I think this is too complicated and fragile. If we want to do something specifically to support the distro use case, I think we should do this: https://reviews.llvm.org/D142174

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?

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