[mlir] ODS: emit interface model method at the end of the header

Previously, ODS interface generator was placing implementations of the
interface's internal "Model" class template immediately after the class
definitions in the header. This doesn't allow this implementation, and
consequently the interface itself, to return an instance of another
interface if its class definition is emitted below. This creates
undesired ordering effects and makes it impossible for two or more
interfaces to return instances of each other. Change the interface
generator to place the implementations of these methods after all
interface classes.

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D136322