Create an array stack type for a shared use-case (#4100)

Based on discussion around the region handling in generic_region_stack,
create a generic structure for the stack-of-vectors support. I also want
to add this to InstBlockStack, but that's a little more complex due to
GlobalInit, so cutting a PR here to check with review.

My work here is how I noticed #4099; I want to be sure that I'm correct
about the issue, but it's the difference between being able to use
PeekArray or not.

Note in scope_stack.h, I believe we could remove next_compile_time_index
and make it just based on elements_size(). However, I want to verify
with you before I make further changes there.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

common/array_stack_test.cpp

@@ -0,0 +1,77 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/array_stack.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+namespace Carbon::Testing {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::IsEmpty;
+
+TEST(ArrayStack, Basics) {
+  ArrayStack<int> stack;
+
+  // PeekAllValues is valid when there are no arrays.
+  EXPECT_THAT(stack.PeekAllValues(), IsEmpty());
+
+  // An array starts empty.
+  stack.PushArray();
+  EXPECT_THAT(stack.PeekArray(), IsEmpty());
+  EXPECT_THAT(stack.PeekAllValues(), IsEmpty());
+
+  // Pushing a couple values works.
+  stack.AppendToTop(1);
+  stack.AppendToTop(2);
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(1, 2));
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2));
+
+  // Pushing a new array starts empty, old values are still there.
+  stack.PushArray();
+  EXPECT_THAT(stack.PeekArray(), IsEmpty());
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2));
+
+  // The added value goes to the 2nd array.
+  stack.AppendToTop(3);
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(3));
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2, 3));
+
+  // Popping goes back to the 1st array.
+  stack.PopArray();
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(1, 2));
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2));
+
+  // Push a couple arrays, then a value on the now-3rd array.
+  stack.PushArray();
+  stack.PushArray();
+  stack.AppendToTop(4);
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(4));
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2, 4));
+
+  // Popping the 3rd array goes to the 2nd array, which is empty.
+  stack.PopArray();
+  EXPECT_THAT(stack.PeekArray(), IsEmpty());
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2));
+
+  // Again back to the 1st array.
+  stack.PopArray();
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(1, 2));
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(1, 2));
+
+  // Go down to no arrays.
+  stack.PopArray();
+  EXPECT_THAT(stack.PeekAllValues(), IsEmpty());
+
+  // Add a new 1st array.
+  stack.PushArray();
+  stack.AppendToTop(5);
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(5));
+  EXPECT_THAT(stack.PeekAllValues(), ElementsAre(5));
+}
+
+}  // namespace
+}  // namespace Carbon::Testing