[mlir][CAPI] Add a simple MlirOpOperand API for MlirValue uses.

This allows basic IR traversal via the C API, which is useful for
analyses in languages other than C++.

This starts by defining an MlirOpOperand struct to encapsulate a pair
of an owner operation and an operand number.

A new API is added for MlirValue, to return the first use of the Value
as an MlirOpOperand, or a "null" MlirOpOperand if there are no uses.

A couple APIs are added for MlirOpOperand. The first checks if an
MlirOpOperand is "null", by checking if its owner's pointer is
null. The second supports iteration along the use-def lists by
accepting an MlirOpOperand and returning the next use of the Value as
another MlirOpOperand, or a "null" MlirOpOperand if there are no more
uses.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D139596