Adding internal C++ dataflow library

cpp/ql/lib/semmle/code/cpp/temp/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 cpp
+
+ /**
+  * DEPRECATED: Use `semmle.code.cpp.dataflow.new.DataFlow` instead.
+  *
+  * Provides classes for performing local (intra-procedural) and
+  * global (inter-procedural) data flow analyses.
+  */
+ module DataFlow {
+   private import semmle.code.cpp.dataflow.internal.DataFlowImplSpecific
+   private import codeql.dataflow.DataFlow
+   import DataFlowMake<CppOldDataFlow>
+   import semmle.code.cpp.dataflow.internal.DataFlowImpl1
+ }