[HLSL] Move DXIL validation version out of ModuleFlags

[HLSL] Move DXIL validation version out of ModuleFlags

Put DXIL validation version into separate NamedMetadata to avoid update ModuleFlags.

Currently DXIL validation version is saved in ModuleFlags in clang codeGen.
Then in DirectX backend, the data will be extracted from ModuleFlags and cause rebuild of ModuleFlags.
This patch will build NamedMetadata for DXIL validation version and remove the code to rebuild ModuleFlags.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D130207