[mlir][tblgen] Consistently use `$_ctxt` instead of `$_ctx`

With the exceptions of AttrOrTypeParameter and DerivedAttr, all of MLIR consistently uses $_ctxt as the substitute variable for the MLIRContext in TableGen C++ code.
Usually this does not matter unless one where to reuse some code in multiple fields but it is still needlessly inconsistent and prone to error.

This patch fixes that by consistently using _$ctxt everywhere.

Differential Revision: https://reviews.llvm.org/D129153