C++: Mark constexpr if as unevalauted.

cpp/ql/lib/semmle/code/cpp/exprs/Expr.qll

@@ -307,6 +307,10 @@ class Expr extends StmtParent, @expr {
     )
     or
     exists(Decltype d | d.getExpr() = this.getParentWithConversions*())
+    or
+    exists(ConstexprIfStmt constIf |
+      constIf.getControllingExpr() = this.getParentWithConversions*()
+    )
   }
 
   /**

cpp/ql/test/library-tests/exprs/unevaluated/unevaluated.expected

@@ -14,3 +14,7 @@
 | test.cpp:57:9:57:9 | 4 |
 | test.cpp:63:9:63:19 | call to getAVirtual |
 | test.cpp:63:9:63:21 | temporary object |
+| test.cpp:67:16:67:26 | sizeof(int) |
+| test.cpp:67:16:67:31 | ... == ... |
+| test.cpp:67:31:67:31 | 4 |
+| test.cpp:67:31:67:31 | (unsigned long)... |