[clang] [clang] constexpr built-in reduce add function. (PR #116243)

[Simon Pilgrim via cfe-commits]

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@@ -723,3 +723,9 @@ not within the bounds of the input vectors; index of -1 found at position 0 is n
 permitted in a constexpr context}}
         vector4charConst1,
         vector4charConst2, -1, -1, -1, -1);
+
+static_assert(__builtin_reduce_add((vector4char){}) == 0);
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RKSimon wrote:

How is this handled? I'd have expected `Source.getVectorElt(0).getInt()` to have failed?

https://github.com/llvm/llvm-project/pull/116243