Use MemRefDescriptor in Linalg-to-LLVM conversion

Following up on the consolidation of MemRef descriptor conversion, update
Linalg-to-LLVM conversion to use the helper class that abstracts away the
implementation details of the MemRef descriptor. This required MemRefDescriptor
to become publicly visible. Since this conversion is heavily EDSC-based,
introduce locally an additional wrapper that uses builder and location pointed
to by the EDSC context while emitting descriptor manipulation operations.

PiperOrigin-RevId: 280429228