[clang] [clang] Add __builtin_start_object_lifetime builtin. (PR #82776)

[A. Jiang via cfe-commits]

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@@ -896,6 +896,12 @@ def Launder : Builtin {
   let Prototype = "void*(void*)";
 }
 
+def StartObjectLifeTime : Builtin {
+  let Spellings = ["__builtin_start_object_lifetime"];
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frederick-vs-ja wrote:

IMO if the intent of this intrinsic is to handle polymorphic classes (which is not covered by `std::start_lifetime_as`), its name should diverge from "plain" `start_lifetime`.

https://github.com/llvm/llvm-project/pull/82776