Enable creation of a LinAlg Matrix (type, intrinsic, dxil op)

[V-FEXrt]

This issue is the very first of the set of issues to support the Linear Algebra spec. Because of this it has to solve the chicken-or-the-egg problem.

We need the new dx.types.MatrixRef type to implement any new intrinsic, but to add the dx.types.MatrixRef type we need an intrinsic to actually observe it. Because of this the following tasks must be completed as an atomic unit.

• Introduce and test the dx.types.MatrixRef type
• Introduce, lower, and test the __builtin_la_CreateMatrix() HLSL intrinsic
• Introduce, lower, and test the dx.op.createMatrix DXIL Op

Testing for the above should over at least the following

• Clang codegen for the intrinsic
• DXIL lowering
• Myriad of tests to ensure correct usage/lowering/validation/error messages for the new HLSL type

#7097 is a good example PR of the type of work that needs to be done.

[damyanp]

From refinement:

• describe the types of tests that'll be written
  • clang codegen
  • dxil lowering
  • additional tests for the new type

There's no mention on the new type in the issue, we want to add the new type simultaneously with adding this op.