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

[Mythreya Kuricheti 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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MythreyaK wrote:

> Why do you need to have yaml files under specific target-triple dirs? It should be possible to flatten them out into a single yaml file.

I'm trying to build a [clang sysroot](https://github.com/MythreyaK/clang-multi-triple) that can support multiple triples in one sysroot. Having the `multilib.yaml` in the root of the sysroot works well for sysroot targeted at a triple-specific cross-compiler build, but I saw that say linux-musl triples place the `multilib.yaml` in the triple-specific directory. So it felt natural (in my inexperienced opinion) to place baremetal `multilib.yaml` in the triple-specific dirs for baremetal too. 

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