[mlir] Finish removing Identifier from the C++ API

There have been a few API pieces remaining to allow for a smooth transition for
downstream users, but these have been up for a few months now. After this only
the C API will have reference to "Identifier", but those will be reworked in a followup.

The main updates are:
* Identifier -> StringAttr
* StringAttr::get requires the context as the first parameter
  - i.e. `Identifier::get("...", ctx)` -> `StringAttr::get(ctx, "...")`

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D116626