C++: Fix off-by-one in \cX handling

Author: jketema

cpp/ql/lib/semmle/code/cpp/regex/RegexTreeView.qll

@@ -654,12 +654,12 @@ private module Impl implements RegexTreeViewSig {
 
     /**
      * Gets the unicode char for this escape.
-     * E.g. for `\cA` this returns "a".
+     * E.g. for `\cA` this returns `0x01`.
      */
     private string getControl() {
       this.isControl() and
       exists(string letter | letter = this.getText().suffix(2).toUpperCase() |
-        result = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".indexOf(letter).toUnicode()
+        result = ("ABCDEFGHIJKLMNOPQRSTUVWXYZ".indexOf(letter) + 1).toUnicode()
       )
     }
   }

cpp/ql/test/library-tests/regex/regexp.expected

@@ -316,8 +316,8 @@ regExpNormalCharValue
 | regexp.cpp:84:39:84:39 | f | f |
 | regexp.cpp:90:19:90:24 | \\u0061 | a |
 | regexp.cpp:93:19:93:22 | \\x61 | a |
-| regexp.cpp:96:21:96:23 | \\cA | \u0000 |
-| regexp.cpp:97:22:97:24 | \\cz | \u0019 |
+| regexp.cpp:96:21:96:23 | \\cA | \u0001 |
+| regexp.cpp:97:22:97:24 | \\cz | \u001a |
 | regexp.cpp:100:20:100:21 | \\0 | \u0000 |
 | regexp.cpp:101:21:101:22 | \\0 | \u0000 |
 | regexp.cpp:104:21:104:21 | a | a |