[clang] [clang][Interp] Fix returning nullptr from functions (PR #67229)

[Shafik Yaghmour via cfe-commits]

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@@ -209,7 +209,7 @@ bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
     // FIXME: We could be calling isLive() here, but the emitted diagnostics
     // seem a little weird, at least if the returned expression is of
     // pointer type.
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shafik wrote:

Can we add to the comment that we are special casing `nullptr` b/c they are not considered alive?

https://github.com/llvm/llvm-project/pull/67229