Introduce OpOperandAdaptors and emit them from ODS

When manipulating generic operations, such as in dialect conversion /
rewriting, it is often necessary to view a list of Values as operands to an
operation without creating the operation itself.  The absence of such view
makes dialect conversion patterns, among others, to use magic numbers to obtain
specific operands from a list of rewritten values when converting an operation.
Introduce XOpOperandAdaptor classes that wrap an ArrayRef<Value *> and provide
accessor functions identical to those available in XOp.  This makes it possible
for conversions to use these adaptors to address the operands with names rather
than rely on their position in the list.  The adaptors are generated from ODS
together with the actual operation definitions.

This is another step towards making dialect conversion patterns specific for a
given operation.

Illustrate the approach on conversion patterns in the standard to LLVM dialect
conversion.

PiperOrigin-RevId: 251232899