Array.concat now takes ConcatArray, not ReadonlyArray (#21462)

* Overloads in Array.concat now handle ReadonlyArray

Previously it was union types, which is slower.

* Make arrayConcat3 test stricter

* Switch to InputArray instead of adding overloads

* Update baselines

* Update baselines correctly

* Rename to ConcatArray and add slice method

Should make it, respectively, easier to understand this specific type
and harder to satisfy it by mistake.

src/lib/es5.d.ts

@@ -992,12 +992,12 @@ interface ReadonlyArray<T> {
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: ConcatArray<T>[]): T[];
     /**
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: (T | ConcatArray<T>)[]): T[];
     /**
       * Adds all the elements of an array separated by the specified separator string.
       * @param separator A string used to separate one element of an array from the next in the resulting String. If omitted, the array elements are separated with a comma.
@@ -1087,6 +1087,13 @@ interface ReadonlyArray<T> {
     readonly [n: number]: T;
 }
 
+interface ConcatArray<T> {
+    readonly length: number;
+    readonly [n: number]: T;
+    join(separator?: string): string;
+    slice(start?: number, end?: number): T[];
+}
+
 interface Array<T> {
     /**
       * Gets or sets the length of the array. This is a number one higher than the highest element defined in an array.
@@ -1113,12 +1120,12 @@ interface Array<T> {
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: ConcatArray<T>[]): T[];
     /**
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: (T | T[] | ReadonlyArray<T>)[]): T[];
+    concat(...items: (T | ConcatArray<T>)[]): T[];
     /**
       * Adds all the elements of an array separated by the specified separator string.
       * @param separator A string used to separate one element of an array from the next in the resulting String. If omitted, the array elements are separated with a comma.