[mlir] support data layout specs on ModuleOp

[mlir] support data layout specs on ModuleOp

ModuleOp is a natural place to provide scoped data layout information. However,
it is undesirable for ModuleOp to implement the entirety of
DataLayoutOpInterface because that would require either pushing the interface
inside the IR library instead of a separate library, or putting the default
implementation of the interface as inline functions in headers leading to
binary bloat. Instead, ModuleOp accepts an arbitrary data layout spec attribute
and has a dedicated hook to extract it, and DataLayout is modified to know
about ModuleOp particularities.

Reviewed By: herhut, nicolasvasilache

Differential Revision: https://reviews.llvm.org/D98500