* [MLIR] Add support for parsing nGraph tensor type

Initial commit that enables nGraph parsing. It's needed for testing.

* [MLIR] Enable nGraph dialect in ngraph-opt

This PR registers nGraph dialect in ngraph-opt and prepares
nGraph lowering pass for LIT testing, fixing all the related issues.
Among other things, lowering pass has to be turned into a function pass,
dead argument in constructor was removed and `convert-ngraph-to-affine`
flag was added.

* Fix issue with function name and multiple functions

* Extend module_function.mlir lit test

* Improve module_function.mlir test

Remove ngraph to affine dialect conversion since we just need to verify
that we can parse and print modules and functions.
Add verification for parsing the printed code.

* [MLIR] Add support for parsing nGraph element types (#3665)

* [MLIR] Add support for parsing nGraph element types

It introduces initial support for parsing nGraph signed/unsigned
integer and floating point data types.

* Improve LIT tests

Test parsing and printing of types separately from lowering to affine
since these tests will evolve differently, particularly for tensor
types.

* Missed file

I left this file behind in the previous commit