[HLSL][DXC Driver] Add preprocessor definition when `-metal` is passed

[llvm-beanz]

Is your feature request related to a problem? Please describe.

We recently added a -metal flag to DXC to aid with cross-platform development workflows. When that flag is passed we should set a new preprocessor define in InitPreprocessor (something like __metal__) to denote that we're targeting Metal. That will allow for code portability to work around differences in behavior between DX, VK and MTL.

We should ensure this implementation aligns with microsoft/DirectXShaderCompiler#7553.