[clang] [HLSL] Add 6.2 SM on half availability for length intrinsic (PR #122337)

[Finn Plummer via cfe-commits]

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@@ -1297,9 +1297,11 @@ float4 lerp(float4, float4, float4);
 ///
 /// Length is based on the following formula: sqrt(x[0]^2 + x[1]^2 + ...).
 
+_HLSL_16BIT_AVAILABILITY(shadermodel, 6.2)
 const inline half length(half X) { return __detail::length_impl(X); }
 const inline float length(float X) { return __detail::length_impl(X); }
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inbelic wrote:

Do we need `_HLSL_AVAILABILITY(shadermodel, 6.2)` on the none half variants?

https://github.com/llvm/llvm-project/pull/122337