C#: Avoid computing CIL strings and non-PDB locations in data flow library

Computing strings and locations for CIL instructions can be quite time consuming.
The CIL `toString()`s are not very helpful in path explanations, and their locations
are only useful when a PDB source file exists. Therefore, produce a simple constant
`toString()`, and restrict locations to those in PDB files.

Author: hvitved

csharp/ql/src/semmle/code/csharp/dataflow/DataFlow.qll

@@ -53,9 +53,19 @@ module DataFlow {
 
     override DotNet::Callable getEnclosingCallable() { result = expr.getEnclosingCallable() }
 
-    override string toString() { result = expr.toString() }
+    override string toString() {
+      result = expr.(Expr).toString()
+      or
+      expr instanceof CIL::Expr and
+      result = "CIL expression"
+    }
 
-    override Location getLocation() { result = expr.getLocation() }
+    override Location getLocation() {
+      result = expr.(Expr).getLocation()
+      or
+      result.getFile().isPdbSourceFile() and
+      result = expr.(CIL::Expr).getALocation()
+    }
   }
 
   /**