[analyzer][NFC] Refactor GenericTaintChecker to use CallDescriptionMap

GenericTaintChecker now uses CallDescriptionMap to describe the possible
operation in code which trigger the introduction (sources), the removal
(filters), the passing along (propagations) and detection (sinks) of
tainted values.

Reviewed By: steakhal, NoQ

Differential Revision: https://reviews.llvm.org/D116025

clang/include/clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h

@@ -84,6 +84,8 @@ public:
     return Eng.getContext();
   }
 
+  const ASTContext &getASTContext() const { return Eng.getContext(); }
+
   const LangOptions &getLangOpts() const {
     return Eng.getContext().getLangOpts();
   }