[mlir] Add EmitC dialect

This upstreams the EmitC dialect and the corresponding Cpp target, both
initially presented with [1], from [2] to MLIR core. For the related
discussion, see [3].

[1] https://reviews.llvm.org/D76571
[2] https://github.com/iml130/mlir-emitc
[3] https://llvm.discourse.group/t/emitc-generating-c-c-from-mlir/3388



Co-authored-by: Jacques Pienaar <jpienaar@google.com>
Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>
Co-authored-by: Oliver Scherf <oliver.scherf@iml.fraunhofer.de>

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D103969