Add value store to be shared across compile stages. (#3311)

This updates lexing to use the data. I'll do checking separately, just
to split changes.

Note the ValueStore structure is also set up such that SemIR::File can
use it for other fields.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/base/value_store_test.cpp

@@ -0,0 +1,72 @@
+// 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 "toolchain/base/value_store.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+namespace Carbon {
+namespace {
+
+using ::testing::Eq;
+using ::testing::Not;
+
+TEST(ValueStore, Integer) {
+  SharedValueStores value_stores;
+  IntegerId id1 = value_stores.integers().Add(llvm::APInt(64, 1));
+  IntegerId id2 = value_stores.integers().Add(llvm::APInt(64, 2));
+
+  ASSERT_TRUE(id1.is_valid());
+  ASSERT_TRUE(id2.is_valid());
+  EXPECT_THAT(id1, Not(Eq(id2)));
+
+  EXPECT_THAT(value_stores.integers().Get(id1), Eq(1));
+  EXPECT_THAT(value_stores.integers().Get(id2), Eq(2));
+}
+
+TEST(ValueStore, Real) {
+  Real real1{.mantissa = llvm::APInt(64, 1),
+             .exponent = llvm::APInt(64, 11),
+             .is_decimal = true};
+  Real real2{.mantissa = llvm::APInt(64, 2),
+             .exponent = llvm::APInt(64, 22),
+             .is_decimal = false};
+
+  SharedValueStores value_stores;
+  RealId id1 = value_stores.reals().Add(real1);
+  RealId id2 = value_stores.reals().Add(real2);
+
+  ASSERT_TRUE(id1.is_valid());
+  ASSERT_TRUE(id2.is_valid());
+  EXPECT_THAT(id1, Not(Eq(id2)));
+
+  const auto& real1_copy = value_stores.reals().Get(id1);
+  EXPECT_THAT(real1.mantissa, Eq(real1_copy.mantissa));
+  EXPECT_THAT(real1.exponent, Eq(real1_copy.exponent));
+  EXPECT_THAT(real1.is_decimal, Eq(real1_copy.is_decimal));
+
+  const auto& real2_copy = value_stores.reals().Get(id2);
+  EXPECT_THAT(real2.mantissa, Eq(real2_copy.mantissa));
+  EXPECT_THAT(real2.exponent, Eq(real2_copy.exponent));
+  EXPECT_THAT(real2.is_decimal, Eq(real2_copy.is_decimal));
+}
+
+TEST(ValueStore, String) {
+  std::string a = "a";
+  std::string b = "b";
+  SharedValueStores value_stores;
+  StringId a_id = value_stores.strings().Add(a);
+  StringId b_id = value_stores.strings().Add(b);
+
+  ASSERT_TRUE(a_id.is_valid());
+  ASSERT_TRUE(b_id.is_valid());
+
+  EXPECT_THAT(a_id, Not(Eq(b_id)));
+  EXPECT_THAT(value_stores.strings().Get(a_id), Eq(a));
+  EXPECT_THAT(value_stores.strings().Get(b_id), Eq(b));
+}
+
+}  // namespace
+}  // namespace Carbon