[MLIR] Change FuncOp assembly syntax to print visibility inline instead of in attrib dict.

- Change syntax for FuncOp to be `func <visibility>? @name` instead of printing the
  visibility in the attribute dictionary.
- Since printFunctionLikeOp() and parseFunctionLikeOp() are also used by other
  operations, make the "inline visibility" an opt-in feature.
- Updated unit test to use and check the new syntax.

Differential Revision: https://reviews.llvm.org/D90859