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

[Nathan Corbyn via cfe-commits]

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@@ -287,6 +287,10 @@ part of the draft specification.
 The elements of a  value of a matrix type are laid out in column-major order
 without padding.
 
+To change memory layout to row major use the `-fmatrix-default-layout` flag.
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cofibrant wrote:

Should be `-fmatrix-memory-layout`

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