C#: Update the ImplicitToStringExpr class to take AssignAddExpr into account.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/commons/Strings.qll

@@ -49,6 +49,16 @@ class ImplicitToStringExpr extends Expr {
       this = add.getOtherOperand(o).stripImplicit()
     )
     or
+    // s1 += s2 for where the left hand side is a string, call an operator +(string, object)
+    exists(AssignAddExpr add, Operator o, Parameter p0, Parameter p1 |
+      o = add.getTarget() and
+      o.getName() = "+" and
+      p0 = o.getParameter(0) and
+      p1 = o.getParameter(1) and
+      p0.getType() instanceof StringType and
+      this = getAnAssignedArgumentOrParam(p1).stripImplicit()
+    )
+    or
     this = any(InterpolatedStringExpr ise).getAnInsert().stripImplicit()
   }
 }