[HLSL] Implement vector extension/truncation/conversion standard conversion sequences

[llvm-beanz]

HLSL's standard conversion sequences support vector splat and truncation as a "second" conversions, and have an element-wise integral or floating point conversion that can be inserted between C++'s second and third conversions.

This is not well documented, but the WIP specification of the behavior is here:

https://github.com/microsoft/hlsl-specs/blob/main/specs/language/conversions.tex

The draft specification is also available as a PDF. The relevant chapter has the stable reference [Conv].

[llvm-beanz]

Merged in #71098

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Chris B (llvm-beanz)

HLSL's standard conversion sequences support vector splat and truncation as a "second" conversions, and have an element-wise integral or floating point conversion that can be inserted between C++'s second and third conversions.

This is not well documented, but the WIP specification of the behavior is here:

https://github.com/microsoft/hlsl-specs/blob/main/specs/language/conversions.tex

The draft specification is also available as a PDF. The relevant chapter has the stable reference [Conv].