QL: QL: Add dataflow library.

Author: MathiasVP

ql/src/codeql/dataflow/DataFlow.qll

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+/**
+ * Provides a library for local (intra-procedural) and global (inter-procedural)
+ * data flow analysis: deciding whether data can flow from a _source_ to a
+ * _sink_.
+ *
+ * Unless configured otherwise, _flow_ means that the exact value of
+ * the source may reach the sink. We do not track flow across pointer
+ * dereferences or array indexing. To track these types of flow, where the
+ * exact value may not be preserved, import
+ * `semmle.code.cpp.dataflow.TaintTracking`.
+ *
+ * To use global (interprocedural) data flow, extend the class
+ * `DataFlow::Configuration` as documented on that class. To use local
+ * (intraprocedural) data flow between expressions, call
+ * `DataFlow::localExprFlow`. For more general cases of local data flow, call
+ * `DataFlow::localFlow` or `DataFlow::localFlowStep` with arguments of type
+ * `DataFlow::Node`.
+ */
+
+import ql
+
+module DataFlow {
+  import internal.DataFlowImpl
+}

ql/src/codeql/dataflow/internal/DataFlowDispatch.qll

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+private import ql
+private import DataFlowUtil
+
+/**
+ * Gets a function that might be called by `call`.
+ */
+DataFlowCallable viableCallable(Call call) { result.asPredicate() = call.getTarget() }
+
+/**
+ * Holds if the set of viable implementations that can be called by `call`
+ * might be improved by knowing the call context.
+ */
+predicate mayBenefitFromCallContext(Call call, DataFlowCallable f) { none() }
+
+/**
+ * Gets a viable dispatch target of `call` in the context `ctx`. This is
+ * restricted to those `call`s for which a context might make a difference.
+ */
+DataFlowCallable viableImplInCallContext(Call call, Call ctx) { none() }