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

[Farzon Lotfi via cfe-commits]

farzonl wrote:

> 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?

The bug isn’t in the construction it was in how we were indexing the matrix. The initialization is the same for both dxc and clang.

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