Make 'getIndentString' just use a loop. (microsoft#41221)

* Make 'getIndentString' just use a loop.

* Added test.

* Accepted baselines.

Author: DanielRosenwasser

src/compiler/utilities.ts

@@ -3849,8 +3849,10 @@ namespace ts {
 
     const indentStrings: string[] = ["", "    "];
     export function getIndentString(level: number) {
-        if (indentStrings[level] === undefined) {
-            indentStrings[level] = getIndentString(level - 1) + indentStrings[1];
+        // prepopulate cache
+        const singleLevel = indentStrings[1];
+        for (let current = indentStrings.length; current <= level; current++) {
+            indentStrings.push(indentStrings[current - 1] + singleLevel);
         }
         return indentStrings[level];
     }