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

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cor3ntin wrote:

I am a bit concerned that this does not actually have the desired semantics at all, but @zygoloid seemed to be "happy" with it.  I will admit I struggle understanding the motivation of adding a builtin that does...less than it should (when it should do something).

Similarly, do you have plans for the array version of start_lifetime_as?

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