[mlirbc] Add IntegerSetAttr (#207482)
Serialize IntegerSetAttr using VarInt for numDims and numSymbols and
arrays for contraints and eqFlags. Add ArrayWithKnownSize` an Array
variant that omits the length prefix when the size is known from a
previously serialized member. This avoids encoding redundant array
lengths when, for example, two arrays are always the same size.
Assisted-By: Gemini
