[HLSL] Implement a header only distance intrinsic

[farzonl]

• For the HLSL intrinsic used const inline because that seems to be the pattern countbits
• For the helper functions the pattern in hlsl_detail.h was to do constexpr like with bit_cast and enable_if_t So did the same here.
• Distance in DXC is defined as Length(X-Y), So doing the same here.
• Maybe this also means we need to move length into the header
• This resolves all the DirectX specific parts of Implement the distance HLSL Function #99107
• The codegen will be functionally correct for SPIRV, but will not emit the GLSL Distance opcode.
• There are many potential solutions for that. Atm, we intend to address the GLSL specific ops with Inline SPIRV.

[damyanp]

Would we want to also put this in namespace __detail, or prefix the _impl versions with __ or something?

[farzonl]

moving implementations to a separate header makes sense. It would simplify our ordering problem and issues with type promotion that can pop up when using the builtins on scalars. There isn't much in the __detail namespace yet so I don't know what the rules are for putting things there.

[damyanp]

It doesn't necessarily need to be moved to a different header to put it in a different namespace.

Wondering how far we can go with having lots of headers....would having one header per-builtin be reasonable? Would precompiled headers totally swallow any per-file overhead there?