[ODS] Make the getType() method on a OneResult instruction return a specific type.

Implement Bug 46698, making ODS synthesize a getType() method that returns a
specific C++ class for OneResult methods where we know that class.  This eliminates
a common source of casts in things like:

   myOp.getType().cast<FIRRTLType>().getPassive()

because we know that myOp always returns a FIRRTLType.  This also encourages
op authors to type their results more tightly (which is also good for
verification).

I chose to implement this by splitting the OneResult trait into itself plus a
OneTypedResult trait, given that many things are using `hasTrait<OneResult>`
to conditionalize various logic.

While this changes makes many many ops get more specific getType() results, it
is generally drop-in compatible with the previous behavior because 'x.cast<T>()'
is allowed when x is already known to be a T.  The one exception to this is that
we need declarations of the types used by ops, which is why a couple headers
needed additional #includes.

I updated a few things in tree to remove the now-redundant `.cast<>`'s, but there
are probably many more than can be removed.

Differential Revision: https://reviews.llvm.org/D93790

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