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

[Brian Favela 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);
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bfavela wrote:

Sorry I missed the earlier PR where this was added, but I don't know how signed integers work here. We only support unsigned in HLSL. Is that a change for clang?
Missing tests as well, if so.

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