[DirectX] Don't put entry name into string table for PSV0 before v3

[llvm-beanz]

Currently the entry name is added to the PSV0 string table regardless of whether or not we're targeting PSV0 v3 where it is needed. We should correct that to generate PSV matching DXC.

[llvmbot]

@llvm/issue-subscribers-bug

Author: Chris B (llvm-beanz)

Currently the entry name is added to the PSV0 string table regardless of whether or not we're targeting PSV0 v3 where it is needed. We should correct that to generate PSV matching DXC.