RequestDelegateGenerator should move to GetInterceptableLocation API

[RikkiGibson]

Roslyn just introduced a new API for creating [InterceptsLocation] attributes in generated code. This uses a new opaque encoding for the location.

We'd like our partners to onboard to this new API as soon as is feasible. (We just merged it so may be a day or two before arcade will start handing out compilers which contain it.)

The old path-line-column approach is going to be deprecated and support removed prior to stable release of the feature.

[captainsafia]

Closed in above PR.