[clang] [Clang] Allow LibBuiltins with placeholder return types to be called (PR #101702)

[via cfe-commits]

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@@ -4895,7 +4895,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
   // Never implicitly instantiate a builtin; we don't actually need a function
   // body.
   if (Function->getBuiltinID() && TSK == TSK_ImplicitInstantiation &&
-      !DefinitionRequired)
+      !DefinitionRequired && !Function->getReturnType()->isUndeducedType())
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Sirraide wrote:

I think this ends up affecting more builtins than just the ones we care about here. I’m not sure that’s a good idea, but I’m also not too familiar w/ builtins.

https://github.com/llvm/llvm-project/pull/101702