Merge pull request #20870 from jketema/exp-arg-fix

C++: `getExpandedArgument` fixes

Author: jketema

cpp/ql/lib/semmle/code/cpp/Compilation.qll

@@ -97,7 +97,7 @@ class Compilation extends @compilation {
   /**
    * Gets an expanded argument passed to the extractor on this invocation.
    */
-  string getAnExpandedArgument() { result = this.getArgument(_) }
+  string getAnExpandedArgument() { result = this.getExpandedArgument(_) }
 
   /**
    * Gets the `i`th expanded argument passed to the extractor on this
@@ -107,7 +107,11 @@ class Compilation extends @compilation {
    * includes the arguments from that file, rather than just taking the
    * argument literally.
    */
-  string getExpandedArgument(int i) { compilation_expanded_args(this, i, result) }
+  string getExpandedArgument(int i) {
+    if exists(string arg | compilation_expanded_args(this, _, arg))
+    then compilation_expanded_args(this, i, result)
+    else result = this.getArgument(i)
+  }
 
   /**
    * Gets the total amount of CPU time spent processing all the files in the