Swift: Fix completion for patterns that occur outside 'CaseLabelItems'. Previously we'd add an unnecessary 'no-match' completion to these patterns even though they were always matching. This caused some confusing output in the graph tests in particular.

Author: MathiasVP

swift/ql/lib/codeql/swift/controlflow/internal/Completion.qll

@@ -265,11 +265,22 @@ private predicate isIrrefutableMatch(Pattern p) {
   isIrrefutableMatch(p.getResolveStep())
 }
 
+/**
+ * Holds if the ast node `ast` is a (possibly top-level) pattern that constantly matches (`value = true`) or
+ * constantly non-matches (`value = false`).
+ */
+private predicate isMatchingConstant(AstNode ast, boolean value) {
+  isMatchingConstantItem(ast, value)
+  or
+  isIrrefutableMatch(ast) and
+  value = true
+}
+
 /**
  * Holds if the top-level pattern `cli` constantly matches (`value = true`) or
  * constantly non-matches (`value = false`).
  */
-private predicate isMatchingConstant(CaseLabelItem cli, boolean value) {
+private predicate isMatchingConstantItem(CaseLabelItem cli, boolean value) {
   // If the pattern always matches
   isIrrefutableMatch(cli.getPattern()) and
   (