Chore: improve performance of SourceCode constructor (#8054)

When generating a list of tokens and comments, the `SourceCode` constructor previously concatenated the list of tokens and the list of comments, and sorted the result. For a file with `n` tokens and `n` comments, this takes `O(nlog(n))` time. However, since the token and comment lists are already sorted, they can be merged in `O(n)` time.

The performance impact of this change is most noticable on large files, since they have a large number of tokens. When linting [test-data.js](https://gist.github.com/mysticatea/2fca6bc8c2194f75e4808a884a1e351a) (48000 lines) with no rules enabled, this improved overall performance by about 20%.

lib/util/source-code.js

@@ -77,6 +77,28 @@ function looksLikeExport(astNode) {
         astNode.type === "ExportAllDeclaration" || astNode.type === "ExportSpecifier";
 }
 
+/**
+ * Merges two sorted lists into a larger sorted list in O(n) time
+ * @param {Token[]} tokens The list of tokens
+ * @param {Token[]} comments The list of comments
+ * @returns {Token[]} A sorted list of tokens and comments
+ */
+function sortedMerge(tokens, comments) {
+    const result = [];
+    let tokenIndex = 0;
+    let commentIndex = 0;
+
+    while (tokenIndex < tokens.length || commentIndex < comments.length) {
+        if (commentIndex >= comments.length || tokenIndex < tokens.length && tokens[tokenIndex].range[0] < comments[commentIndex].range[0]) {
+            result.push(tokens[tokenIndex++]);
+        } else {
+            result.push(comments[commentIndex++]);
+        }
+    }
+
+    return result;
+}
+
 
 //------------------------------------------------------------------------------
 // Public Interface
@@ -117,9 +139,7 @@ function SourceCode(text, ast) {
      */
     this.lines = SourceCode.splitLines(this.text);
 
-    this.tokensAndComments = ast.tokens
-        .concat(ast.comments)
-        .sort((left, right) => left.range[0] - right.range[0]);
+    this.tokensAndComments = sortedMerge(ast.tokens, ast.comments);
 
     // create token store methods
     const tokenStore = new TokenStore(ast.tokens, ast.comments);