Make namespace handling uniform across dialect backends.

Now backends spell out which namespace they want to be in, instead of relying on
clients #including them inside already-opened namespaces. This also means that
cppNamespaces should be fully qualified, and there's no implicit "::mlir::"
prepended to them anymore.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D86811