[clang] fix OutputSemantic list in HLSL (#185550)
Normally sane front-ends with the common calling-conventions avoid
having multiple sret with a return value, so this is NFCI. However,
multiple can be valid. This rewrites an odd looking DenseMap of one
element that was needed for iteration into a more sensible vector.
Noted in https://github.com/llvm/llvm-project/pull/181740 review.
