[MLIR] Specify namespace for standard dialect using cppNamespace field

The `::mlir` namespace for operations from standard is currently
defined by enclosing the header file generated from the ODS in
`Ops.td` in a namespace in `Ops.h`. However, when referencing
operations from `Ops.td` in other TableGen files, this causes the
generated C++ code to refer to classes from the global namespace
instead of `::mlir`.

By defining the namespace through the `cppNamespace` field for
`StandardOps_Dialect` directly in `Ops.td` instead, the ODS
becomes reusable in other TableGen files through simple
inclusion.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D103234