Improvements and changes in dialect conversion APIs. Type conversion
and op rewriting are now a bit more independent mechanisms. Dialect
conversion now requires a target object with the legal transformations
to be applied. Legality checks to detect illegal operations after
dialect conversion have been strengthened significantly.

MLIR LLVM commit:
commit 82d5084cf28bca7a0eebec283cdeae118dfa1d11 (HEAD)
Author: Jacques Pienaar <jpienaar@google.com>
Date:   Wed Jun 26 11:12:40 2019 -0700

    Split out TranslateClParser and add new parse method that reuses
    SourceMgr.

MLIR commit:
commit 82d5084cf28bca7a0eebec283cdeae118dfa1d11 (HEAD)
Author: Jacques Pienaar <jpienaar@google.com>
Date:   Wed Jun 26 11:12:40 2019 -0700

    Split out TranslateClParser and add new parse method that reuses
    SourceMgr.

Add support for any rank ArgMin. Hacky workaround for IndexType tensors by doing type-conversion after operation

* Initial td file. Cmake changes

* Move all ops to .td file.

* Added few more opcodes to show-case. Fixed PR feedback

* Remove NG_ prefix of opcode records. Some fixes

* Added some doc

*  Adding back NG prefix

* Bug fix in MLIR gen

* Move MLIR to src/contrib/mlir

* Create MLIR as cmake external project. Clone and build via ngraph cmake

* Moved code and enabled compilation. Need to clone and build MLIR/LLVM during cmake step, so find_package can work

* clone and build llvm/mlir during configuration. Compiles now. Needs more testing

* Force DEX only if MLIR is ON

* Remove extra cmake file. Style-apply

* Remove redundant files in cpu/mlir

* Update CODEOWNERS. Check for ninja and fail if not found

* Fixing post merge issues