[mlir] support translation of multidimensional vectors to LLVM IR

[mlir] support translation of multidimensional vectors to LLVM IR

Summary:
MLIR unlike LLVM IR supports multidimensional vector types. Such types are
lowered to nested LLVM IR arrays wrapping an LLVM IR vector for the innermost
dimension of the MLIR vector. MLIR supports constants of such types using
ElementsAttr for values. Introduce support for converting ElementsAttr into
LLVM IR Constant Aggregates recursively. This enables translation of
multidimensional vector constants from MLIR to LLVM IR.

Differential Revision: https://reviews.llvm.org/D72846