[QuantOps] Add the quant region definition

Summary:
This regional op in the QuantOps dialect will be used to wrap
high-precision ops into atomic units for quantization. All the values
used by the internal ops are captured explicitly by the op inputs. The
quantization parameters of the inputs and outputs are stored in the
attributes.

Subscribers: jfb, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75972