Apply suggestions from code review

Co-authored-by: Chris Smowton <smowton@github.com>

Author: JLLeitschuh

java/ql/lib/change-notes/2022-03-07-char-literal-in-compile-time-constant.md

@@ -1,5 +1,5 @@
 ---
 category: minorAnalysis
 ---
- * Add new method `CharacterLiteral.getStringifiedValue` which attempts to compute the
-   `String.valueOf` compile time constant value of the expression.
+ * Add new method `CompileTimeConstantExpr.getStringifiedValue` which attempts to compute the
+   `String.valueOf` string rendering of a constant expression.

java/ql/lib/semmle/code/java/Expr.qll

@@ -201,17 +201,9 @@ class CompileTimeConstantExpr extends Expr {
     result = this.(StringLiteral).getValue()
     or
     (
-      if this.(AddExpr).getAnOperand().getType() instanceof TypeString // If either operand type is already a String:
-      then
-        // Then the stringified version of the expression can be safely used as a String will be created.
-        result =
-          this.(AddExpr).getLeftOperand().(CompileTimeConstantExpr).getStringifiedValue() +
-            this.(AddExpr).getRightOperand().(CompileTimeConstantExpr).getStringifiedValue()
-      else
-        // Adding two literals of primitive type will not result in a String.
-        result =
-          this.(AddExpr).getLeftOperand().(CompileTimeConstantExpr).getStringValue() +
-            this.(AddExpr).getRightOperand().(CompileTimeConstantExpr).getStringValue()
+      this.getType() instanceof TypeString and
+      this.(AddExpr).getLeftOperand().(CompileTimeConstantExpr).getStringifiedValue() +
+        this.(AddExpr).getRightOperand().(CompileTimeConstantExpr).getStringifiedValue()
     )
     or
     // Ternary conditional, with compile-time constant condition.