Add support for Variable.is_constinit()

Author: Matthew Gretton-Dann

cpp/ql/src/semmle/code/cpp/Variable.qll

@@ -144,6 +144,11 @@ class Variable extends Declaration, @variable {
    */
   predicate isConstexpr() { this.hasSpecifier("is_constexpr") }
 
+  /**
+   * Holds if this variable is declared `constinit`.
+   */
+  predicate isConstinit() { this.hasSpecifier("declared_constinit") }
+
   /**
    * Holds if this variable is `thread_local`.
    */