[clang] [Matrix] Add a row\col major toggle in the clang driver (PR #167628)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

> I'm still struggling to understand the motivation for this change and the broader program of changes you keep bringing up. As far as I understand, there are three separate issues at play which we seem to be conflating:
> 
> 1. How matrix types are exposed in the Clang front-end (e.g., whether an initialiser list is treated as row / column major);
> 2. How Clang lays out matrix types in memory;
> 3. How LLVM lowers use of matrix builtins to vector instructions.
> 
> In my opinion, it does not make much sense for a compiler flag to impact 1, and it makes perfect sense for both 2 and 3 to vary independently of 1 under the hood. If row major memory layout is important and a priority, work to implement the corresponding LLVM intrinsics should come first. It seems very premature to add to, e.g., the release notes that we're allowing users to control matrix memory layouts when nothing in the compiler has changed to actually support these.

@cofibrant  Ok here is my plan for unblocking this pr:

@fhahn  mentioned:
> I think you have to also prevent using builtin_column_major_load/store (or preferably update LowerMatrixIntrinsics), because currently using them in row-major mode causes a crash: https://clang.godbolt.org/z/1Gd41Gx7P

I will add back the -mllvm flags and use the clang flag to enable or disable usage of `__builtin_matrix_column_major_load` and `__builtin_matrix_column_major_store` based on this flag. That at least will let this flag prevent a crash.

- I'll hold off on any changes that impact initalizer lists.  
- Hopefully we can move forward with MatrixSubscriptExpr indexing changes though?
- I'll move forward with clang-dxc driver changes that will let me map /Zpr and /Zpc to the equivalent column/row major order flag. 




https://github.com/llvm/llvm-project/pull/167628