[clang] [llvm] [HLSL] Re-implement countbits with the correct return type (PR #113189)

[Simon Pilgrim via cfe-commits]

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@@ -705,66 +705,74 @@ float4 cosh(float4);
 
 #ifdef __HLSL_ENABLE_16_BIT
 _HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_elementwise_popcount)
-int16_t countbits(int16_t);
+constexpr uint countbits(int16_t x) {
+  return __builtin_elementwise_popcount(x);
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RKSimon wrote:

`__builtin_elementwise_popcount` isn't currently a constexpr builtin

https://github.com/llvm/llvm-project/pull/113189