[mlir] enable delayed registration of dialect interfaces

This introduces a mechanism to register interfaces for a dialect without making
the dialect itself depend on the interface. The registration request happens on
DialectRegistry and, if the dialect has not been loaded yet, the actual
registration is delayed until the dialect is loaded. It requires
DialectRegistry to become aware of the context that contains it and the context
to expose methods for querying if a dialect is loaded.

This mechanism will enable a simple extension mechanism for dialects that can
have interfaces defined outside of the dialect code. It is particularly helpful
for, e.g., translation to LLVM IR where we don't want the dialect itself to
depend on LLVM IR libraries.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D96137

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