[mlir] NFC Async: always use 'b' for the current builder

Currently some of the nested IR building inconsistently uses `nb` and `b`, it's very easy to call wrong builder outside of the current scope, so for simplicity all builders are always called `b`, and in nested IR building regions they just shadow the "parent" builder.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D120003