[mlir] Make MemRef element type extensible

Historically, MemRef only supported a restricted list of element types that
were known to be storable in memory. This is unnecessarily restrictive given
the open nature of MLIR's type system. Allow types to opt into being used as
MemRef elements by implementing a type interface. For now, the interface is
merely a declaration with no methods. Later, methods to query, e.g., the type
size or whether a type can alias elements of another type may be added.

Harden the "standard"-to-LLVM conversion against memrefs with non-builtin
types.

See https://llvm.discourse.group/t/rfc-memref-of-custom-types/3558.

Depends On D103826

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D103827