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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> The Fedora distributors were the only group that were against doing this by default, so the suggestion is that Fedora uses -DCLANG_ALLOW_IMPLICIT_RPATH=OFF when building.

I think other distributors and I are against this as well.

What this PR does can be achieved by adding -frtlib-add-rpath to `bin/clang.cfg`.

We have moved away from adding random `CLANG_*`. The configuration file is exactly designed for such purposes.

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I don't think clang driver should be in the position to understand a thousand file architectures.
This is simply not scalable.


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