[mlir] Cleanup: Fix warnings in MLIR

[mlir] Cleanup: Fix warnings in MLIR

Tested with gcc-10. Other compilers may generate additional warnings. This does not fix all warnings. There are a few extra ones in LLVMCore and MLIR.

* `OpEmitter::getAttrNameIndex`: -Wunused-function (function is private and not used anywhere)
* `PrintOpPass` copy constructor: -Wextra ("Base class should be explicitly initialized in the copy constructor")
* `LegalizeForLLVMExport.cpp`: -Woverflow (overflow is expected, silence warning by making the cast explicit)

Differential Revision: https://reviews.llvm.org/D107525