[clang] [clang]: baremetal `multilib.yaml` discovery (PR #227965)

[Sam Elliott via cfe-commits]

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@@ -202,10 +202,10 @@ ToolChain::findMultilibsYAML(const llvm::opt::ArgList &Args, const Driver &D,
   }
 
   SmallString<128> MultilibPath;
-  if (!FallbackDir.empty())
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lenary wrote:

Ok, I haven't had time to dig deeply into the code.

I have some questions:

- are both the `triple/multilib.yaml` and `multilib.yaml` loaded, or only the first one found?
- in the baremetal `multilib.yaml`, right now the main paths in the config are relative to the single `multilib.yaml` itself (which is in a top-level directory), rather than applying somewhere else (though resource dir path management is different and not very well tested). What's the plan here in the per-triple directories?

https://github.com/llvm/llvm-project/pull/227965