C/C++ Overlay: Preserve entities that have at least one location in an unchanged file

Previously, an entity would be discarded if it had any location in a changed
file. This caused issues for entities with multiple declaration entries, such
as extern variables declared in one file and defined in another.

For example, given:
  // a.c (changed)       // b.c (unchanged)
  extern int x;          int x;

The variable `x` should be preserved because it has a location in the
unchanged file b.c, even though it also has a location in the changed file a.c.

cpp/ql/lib/semmle/code/cpp/internal/Overlay.qll

@@ -16,25 +16,49 @@ private string getLocationFilePath(@location_default loc) {
   exists(@file file | locations_default(loc, file, _, _, _, _) | files(file, result))
 }
 
-/**
- * Gets the file path for an element in the base variant.
- */
 overlay[local]
-private string getElementPathInBase(@element e) {
-  not isOverlay() and
-  exists(@location_default loc |
-    // Direct location (declarations)
-    var_decls(e, _, _, _, loc)
-    or
-    // Indirect location (entities)
-    exists(@var_decl vd | var_decls(vd, e, _, _, _) | var_decls(vd, _, _, _, loc))
-  |
-    result = getLocationFilePath(loc)
-  )
+private class DiscardableEntityBase extends @element {
+  /** Gets the path to the file in which this element occurs. */
+  abstract string getFilePath();
+
+  /** Holds if this element exists in the base variant. */
+  predicate existsInBase() { not isOverlay() }
+
+  /** Gets a textual representation of this discardable element. */
+  string toString() { none() }
 }
 
 /**
- * Discard any element from the base that is in a changed file.
+ * Discard an entity from the base if all its locations are in changed files.
+ * Entities with at least one location in an unchanged file are kept.
  */
 overlay[discard_entity]
-private predicate discardElement(@element e) { overlayChangedFiles(getElementPathInBase(e)) }
+private predicate discardEntity(@element e) {
+  e =
+    any(DiscardableEntityBase de |
+      de.existsInBase() and
+      overlayChangedFiles(de.getFilePath()) and
+      // Only discard if ALL file paths are in changed files
+      forall(string path | path = de.getFilePath() | overlayChangedFiles(path))
+    )
+}
+
+/** A discardable variable declaration entry. */
+overlay[local]
+private class DiscardableVarDecl extends DiscardableEntityBase instanceof @var_decl {
+  override string getFilePath() {
+    exists(@location_default loc | var_decls(this, _, _, _, loc) |
+      result = getLocationFilePath(loc)
+    )
+  }
+}
+
+/** A discardable variable. */
+overlay[local]
+private class DiscardableVariable extends DiscardableEntityBase instanceof @variable {
+  override string getFilePath() {
+    exists(@var_decl vd, @location_default loc | var_decls(vd, this, _, _, loc) |
+      result = getLocationFilePath(loc)
+    )
+  }
+}