[clang] [llvm] Add length HLSL function to DirectX Backend (PR #101256)

[Farzon Lotfi via cfe-commits]

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@@ -249,6 +249,7 @@ DWARF Support in Clang
 
 Floating Point Support in Clang
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+- Add ``__builtin_elementwise_length``builtin for floating point types only.
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farzonl wrote:

Same comment this doesn't match `__builtin_hlsl_elementwise_length`. Also These release notes are probably not intended for the HLSL dialect of clang.  Should we remove this line?

https://github.com/llvm/llvm-project/pull/101256