[HLSL][SPIRV] Restore support for -g to generate NSDI (#190007)
The original attempt (#187051) produced a regression for
`intel-sycl-gpu` because `SPIRVEmitNonSemanticDI` will now self-activate
whenever `llvm.dbg.cu` is present. This removed the need for the
explicit `--spv-emit-nonsemantic-debug-info` flag.
The pass is now entered unconditionally for all SPIR-V targets, but
`NonSemantic.Shader.DebugInfo.100` requires the
`SPV_KHR_non_semantic_info`. Targets like `spirv64-intel` do not enable
that extension by default. When `checkSatisfiable()` ran on those
targets, it issued a fatal error rather than silently skipping.
Adds an early-out from `emitGlobalDI()`: if
`SPV_KHR_non_semantic_info` is not available for the current target, the
pass returns without emitting anything.