[mlir][ArmSME] Introduce custom TypeConverter for ArmSME

[mlir][ArmSME] Introduce custom TypeConverter for ArmSME

At the moment, SME-to-LLVM lowerings rely entirely on
`LLVMTypeConverter`. This patch introduces a dedicated `TypeConverter`
that inherits from `LLVMTypeConverter` (it will also be used when
lowering ArmSME Ops to LLVM).

The new type converter merely disables lowerings for `VectorType` to
prevent 2-d scalable vectors (common in the context of ArmSME), e.g.

   `vector<[16]x[16]xi8>`,

entering the LLVM Type converter. LLVM does not support arrays of
scalable vectors and hence the need for specialisation. In the case of
SME such types are effectively eliminated when emitting LLVM IR
intrinsics for SME.

Differential Revision: https://reviews.llvm.org/D155365