The Itanium CXX ABI grammer has been extended to support parameterized
vendor extended types [1].
This patch updates Clang's mangling for matrix types to use the new
extension.
[1] https://github.com/itanium-cxx-abi/cxx-abi/commit/b359d28971bdb961dd9b61bd0ef8c884452b4740
"Mangle matrix types as a vendor extended type:"