[mlir][LLVMIR] Add support for importing struct-type ConstantAggregate(Zero)

And move importer test files from `test/Target/LLVMIR` into
`test/Target/LLVMIR/Import`.

We simply translate struct-type ConstantAggregate(Zero) into a
serious of `llvm.insertvalue` operations against a `llvm.undef` root.
Note that this doesn't affect the original logics on translating
vector/array-type ConstantAggregate values.

Differential Revision: https://reviews.llvm.org/D124399