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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Could we have the upstream default be to install a clang.cfg file in bin/ which has -fimplicit-openmp-rpath (or whatever the hoption is)?

Configuration files as far as I'm aware can't do toolchain / language specific stuff, so it's a little weaker than being able to just restrict this to `-fopenmp`. I remember this being brought up as a solution, but in the Fedora case it seemed to be equivalent. At least that was my understanding of Fedora's needs for `-rpath` handling. I'd be totally fine with a default installed clang config file if people are okay with it setting the -rpath. I'm assuming clang config files know things like the triple so it can actually look in places like `/lib/x86...`?

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