[mlir] Support setting operand values in C and Python APIs.

This adds `mlirOperationSetOperand` to the IR C API, similar to the
function to get an operand.

In the Python API, this adds `operands[index] = value` syntax, similar
to the syntax to get an operand with `operands[index]`.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D101398