[clang] [clang][Driver] Add a custom error option in multilib.yaml. (PR #105684)

[Fangrui Song via cfe-commits]

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@@ -130,7 +143,7 @@ class MultilibSet {
 
   /// Select compatible variants, \returns false if none are compatible
   bool select(const Multilib::flags_list &Flags,
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MaskRay wrote:

For other parts of Driver, I think `const Driver &` is more commonly used as the first parameter

https://github.com/llvm/llvm-project/pull/105684