C/C++ overlay: Add basic Overlay.qll file

cpp/ql/lib/cpp.qll

@@ -74,3 +74,4 @@ import semmle.code.cpp.Preprocessor
 import semmle.code.cpp.Iteration
 import semmle.code.cpp.NameQualifiers
 import DefaultOptions
+private import semmle.code.cpp.internal.Overlay

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

@@ -0,0 +1,40 @@
+/**
+ * Defines entity discard predicates for C++ overlay analysis.
+ */
+
+/**
+ * Holds always for the overlay variant and never for the base variant.
+ * This local predicate is used to define local predicates that behave
+ * differently for the base and overlay variant.
+ */
+overlay[local]
+predicate isOverlay() { databaseMetadata("isOverlay", "true") }
+
+/** Gets the file path for a location. */
+overlay[local]
+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)
+  )
+}
+
+/**
+ * Discard any element from the base that is in a changed file.
+ */
+overlay[discard_entity]
+private predicate discardElement(@element e) { overlayChangedFiles(getElementPathInBase(e)) }