[clang] [HLSL][LongVec] enable dot product in the frontend (PR #216418)

[Alex Sepkowski via cfe-commits]

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@@ -33,6 +33,10 @@ int test_dot_int3(int3 p0, int3 p1) { return dot(p0, p1); }
 // CHECK: ret i32 %hlsl.dot
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alsepkow wrote:

Does it not make sense to add test cases for 16bit and 64bit cases right now?

https://github.com/llvm/llvm-project/pull/216418