Implement preview cooperative vectors API

[damyanp]

The API described in Proposal 0031 needs to have an implementation added, with tests.

A prototype can be found here: https://github.com/damyanp/DirectXShaderCompiler/tree/cv_api_staging/tools/clang/test/CodeGenDXIL/hlsl/linalg

Open Questions

Distributing the header

For the preview it is fine to require developers to do something like #include <linalg.h>, but we still need to figure out how package this header file up with the preview package.