[clang] [llvm] [HLSL][Matrix] Change MatrixSubscriptExpr flattened index to row major (PR #165666)

[Deric C. via llvm-commits]

https://github.com/Icohedron commented:

In the PR description you give the example of `int2x3 M = int2x3(1, 2, 3, 4, 5, 6); ` but this example or similar does not appear to be in any of the tests for this PR.

Should there be a test here for matrix initialization via the constructor and/or initializer list?
Or does the matrix constructor test in the offload test suite suffice?

https://github.com/llvm/llvm-project/pull/165666