[HLSL][clang][Driver] Support target profile command line option.

The target profile option(/T) decide the shader model when compile hlsl.
The format is shaderKind_major_minor like ps_6_1.
The shader model is saved as llvm::Triple is clang/llvm like
dxil-unknown-shadermodel6.1-hull.
The main job to support the option is translating ps_6_1 into
shadermodel6.1-pixel.
That is done inside tryParseProfile  at HLSL.cpp.

To integrate the option into clang Driver, a new DriverMode DxcMode is
created. When DxcMode is enabled, OSType for TargetTriple will be
forced into Triple::ShaderModel. And new ToolChain HLSLToolChain will
be created when OSType is Triple::ShaderModel.

In HLSLToolChain, ComputeEffectiveClangTriple is overridden to call
tryParseProfile when targetProfile option is set.

To make test work, Fo option is added and .hlsl is added for active
-xhlsl.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D122865

Patch by: Xiang Li <python3kgae@outlook.com>