[mlir] Revise naming of MLIROptMain and MLIRMlirOptLib

* Rename CMake target MLIROptMain to MLIROptLib:
   The target provides the main library
* Rename CMake target MLIRMlirOptLib to MLIRMlirOptMain:
   The target provides the main() entry function

At the moment, the Bazel configuration of TenorFlow maps the target
MlirOptLib to "lib/Support/MlirOptMain.cpp" and MlirOptMain to
"tools/mlir-opt/mlir-opt.cpp". This is the other way around in the CMake
configuration. As discussed in the context of the pull request
https://github.com/tensorflow/tensorflow/pull/36301, it seems useful to
revise the naming in the MLIR repo.

Differential Revision: https://reviews.llvm.org/D73778