[mlir] FunctionOpInterface: make get/setFunctionType interface methods

This patch removes the concept of a `function_type`-named type attribute
as a requirement for implementors of FunctionOpInterface. Instead, this
type should be provided through two interface methods, `getFunctionType`
and `setFunctionTypeAttr` (*Attr because functions may use different
concrete function types), which should be automatically implemented by
ODS for ops that define a `$function_type` attribute.

This also allows FunctionOpInterface to materialize function types if
they don't carry them in an attribute, for example.

Importantly, all the function "helper" still accept an attribute name to
use in parsing and printing functions, for example.

Reviewed By: rriddle, lattner

Differential Revision: https://reviews.llvm.org/D139447