[clang] [Clang][Driver] Expose relocation model as multilib flags (PR #149132)

[Simon Tatham via cfe-commits]

statham-arm wrote:

> But I'm not _very_ confident of what an "effective triple" is at all, and I don't know for sure that it's right to call `ComputeEffectiveClangTriple(Args)` to make one.

Answering my own question: I had failed to spot that `ToolChain::getMultilibFlags` was _already_ calling `ComputeEffectiveClangTriple` at the top of the function! So I'm now more confident that it's sensible, but also, I could have just reused the existing value :slightly_smiling_face:

https://github.com/llvm/llvm-project/pull/149132