[dataflowengineoss] composable semantics

[xavierpinho]

• Allows one to compose arbitrary Semantics at runtime, cf. the newly introduced Semantics.after, with NoSemantics as the identity.
• Identifies NilSemantics, which under the current engine seems to work as "negative semantics", removing any potential flows that would otherwise be created. I can't say I have it fully covered yet, but fwiw that's my interpretation at the moment.
• Introduces NoCrossTaintSemantics, which can be built both extensionally/intensionally, allowing one to, for instance, specify that an entire external library should have this kind of semantics.

pysrc2cpg was the frontend I used for testing them, but only because that's the frontend I was playing with. There's no other reason for not testing with other frontends.

[DavidBakerEffendi]

Identifies NilSemantics, which under the current engine seems to work as "negative semantics", removing any potential flows that would otherwise be created. I can't say I have it fully covered yet, but fwiw that's my interpretation at the moment.

The current engine assumes that, if a semantic flow entry is present, any combination of arg flows not explicitly stated is "killed". So I believe this is the correct interpretation, and a good distinction against NoSemantics. If semantics/CALL edges are not present, then all flows are assumed present for that call.