[mlir] Disentangle dialect and extension registrations.

This revision avoids the registration of dialect extensions in Pass::getDependentDialects.

Such registration of extensions can be dangerous because `DialectRegistry::isSubsetOf` is
always guaranteed to return false for extensions (i.e. there is no mechanism to track
whether a lambda is already in the list of already registered extensions).
When the context is already in a multi-threaded mode, this is guaranteed to assert.

Arguably a more structured registration mechanism for extensions with a unique ExtensionID
could be envisioned in the future.

In the process of cleaning this up, multiple usage inconsistencies surfaced around the
registration of translation extensions that this revision also cleans up.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D157703