C++: Move Expr location workaround to Expr.qll

This workaround from `DataFlowUtil.qll` should be useful for any query
that selects an `Expr`. In particular, it's useful for IR data flow.

This commit does not include test changes.

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowUtil.qll

@@ -114,33 +114,12 @@ class ExprNode extends Node, TExprNode {
 
   override string toString() { result = expr.toString() }
 
-  override Location getLocation() {
-    result = getExprLocationOverride(expr)
-    or
-    not exists(getExprLocationOverride(expr)) and
-    result = expr.getLocation()
-  }
+  override Location getLocation() { result = expr.getLocation() }
 
   /** Gets the expression corresponding to this node. */
   Expr getExpr() { result = expr }
 }
 
-/**
- * Gets a location for `e` that's more accurate than `e.getLocation()`, if any.
- */
-private Location getExprLocationOverride(Expr e) {
-  // Base case: the parent has a better location than `e`.
-  e.getLocation() instanceof UnknownExprLocation and
-  result = e.getParent().getLocation() and
-  not result instanceof UnknownLocation
-  or
-  // Recursive case: the parent has a location override that's better than what
-  // `e` has.
-  e.getLocation() instanceof UnknownExprLocation and
-  result = getExprLocationOverride(e.getParent()) and
-  not result instanceof UnknownLocation
-}
-
 abstract class ParameterNode extends Node, TNode {
   /**
    * Holds if this node is the parameter of `c` at the specified (zero-based)

cpp/ql/src/semmle/code/cpp/exprs/Expr.qll

@@ -53,7 +53,32 @@ class Expr extends StmtParent, @expr {
   Element getParent() { exprparents(underlyingElement(this), _, unresolveElement(result)) }
 
   /** Gets the location of this expression. */
-  override Location getLocation() { exprs(underlyingElement(this), _, result) }
+  override Location getLocation() {
+    result = this.getExprLocationOverride()
+    or
+    not exists(this.getExprLocationOverride()) and
+    result = this.getDbLocation()
+  }
+
+  /**
+   * Gets a location for this expression that's more accurate than
+   * `getDbLocation()`, if any.
+   */
+  private Location getExprLocationOverride() {
+    // Base case: the parent has a better location than `this`.
+    this.getDbLocation() instanceof UnknownExprLocation and
+    result = this.getParent().(Expr).getDbLocation() and
+    not result instanceof UnknownLocation
+    or
+    // Recursive case: the parent has a location override that's better than
+    // what `this` has.
+    this.getDbLocation() instanceof UnknownExprLocation and
+    result = this.getParent().(Expr).getExprLocationOverride() and
+    not result instanceof UnknownLocation
+  }
+
+  /** Gets the location of this expressions, raw from the database. */
+  private Location getDbLocation() { exprs(underlyingElement(this), _, result) }
 
   /** Holds if this is an auxiliary expression generated by the compiler. */
   predicate isCompilerGenerated() {