[mlir] Forward arguments of `pair` in `SubElementInterface::replaceImmediateSubElementsImpl`

`SubElementInterface::replaceImmediateSubElementsImpl` specializes tuples so that
arguments are forwarded to type getter. However currently pairs are not supported even though
an example in documents uses a pair as a key type. This patch adds support for pairs as well.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D155043