[MLIR] Refactor memref type -> LLVM Type conversion

- Eliminate duplicated information about mapping from memref -> its descriptor fields
  by consolidating that mapping in two functions:  getMemRefDescriptorFields and
  getUnrankedMemRefDescriptorFields.
- Change convertMemRefType() and convertUnrankedMemRefType() to use these
  functions.
- Remove convertMemrefSignature and convertUnrankedMemrefSignature.

Differential Revision: https://reviews.llvm.org/D90707