[HLSL][clang][Driver] Support validator version command line option.

The DXIL validator version option(/validator-version) decide the validator version when compile hlsl.
The format is major.minor like 1.0.

In normal case, the value of validator version should be got from DXIL validator. Before we got DXIL validator ready for llvm/main, DXIL validator version option is added first to set validator version.

It will affect code generation for DXIL, so it is treated as a code gen option.

A new member std::string DxilValidatorVersion is added to clang::CodeGenOptions.

Then CGHLSLRuntime is added to clang::CodeGenModule.
It is used to translate clang::CodeGenOptions::DxilValidatorVersion into a ModuleFlag under key "dx.valver" at end of clang code generation.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D123884