[mlir] Change the syntax of AffineMapAttr and IntegerSetAttr to avoid...

[mlir] Change the syntax of AffineMapAttr and IntegerSetAttr to avoid conflicts with function types.

Summary: The current syntax for AffineMapAttr and IntegerSetAttr conflict with function types, making it currently impossible to round-trip function types(and e.g. FuncOp) in the IR. This revision changes the syntax for the attributes by wrapping them in a keyword. AffineMapAttr is wrapped with `affine_map<>` and IntegerSetAttr is wrapped with `affine_set<>`.

Reviewed By: nicolasvasilache, ftynse

Differential Revision: https://reviews.llvm.org/D72429