Clarification: DenormFlushToZero sign preservation

[SPV_KHR_float_controls](https://github.khronos.org/SPIRV-Registry/extensions/KHR/SPV_KHR_float_controls.html) does not specify if denorms that are flushed to zero preserve their sign bit or always flush to positive zero.

There is a discussion on the LLVM forums about how to map LLVM's float modes correctly to SPIRV:
https://discourse.llvm.org/t/rfc-hlsl-spirv-denormal-handling-executionmode-emission-in-clang-dxc/91143/3

We're currently intending to use sign-preserving flush-to-zero in the compiler to map to DenormFlushToZero. If a backend instead uses positive zero, this could cause subtle behavior differences when code is constant-evaluated in the frontend vs the backend.

Read the SPV_KHR_float_controls specification and the linked LLVM forum discussion first, focusing on DenormFlushToZero and sign handling for flushed denorms. Determine the intended behavior and update the specification so the sign-preservation rule is explicit and consistent with the compiler mapping concerns described in the issue.

Written by the indexing model from the issue text.