Refactor ValueStore to use custom iterators and ranges (#7506)

Replace the llvm::map_range based anonymous ranges returned by values()
and enumerate() with custom range and iterator types. This avoids
exposing complex template return types and improves encapsulation. But
most importantly, it is *much* cheaper to compile.

This change:
- Defines ValueStoreIterator, ValueStoreEnumerateIterator, and
ValueStoreRange templates in the Internal namespace.
- Exposes them via aliases in ValueStore: Iterator, ConstIterator,
Range, MutableRange, and EnumerateRange.
- Supports safe implicit conversion from Iterator to ConstIterator.
- Supports operator-> even when the value type is returned by value
(e.g., llvm::StringRef) by conditionally const-qualifying the pointer
type.
- Ensures C++20 comparison consistency with custom operator<=> and
operator== definitions.
- Restricts construction of these iterators and ranges to ValueStore
methods by using private constructors and friend declarations.

Assisted-by: Antigravity with Gemini

toolchain/base/value_store_test.cpp

@@ -9,6 +9,7 @@
 
 #include <string>
 
+#include "llvm/ADT/StringRef.h"
 #include "toolchain/base/value_ids.h"
 
 namespace Carbon::Testing {
@@ -44,5 +45,27 @@ TEST(ValueStore, Real) {
   EXPECT_THAT(real2.is_decimal, Eq(real2_copy.is_decimal));
 }
 
+TEST(ValueStore, StringRefIteratorArrow) {
+  ValueStore<IdentifierId, llvm::StringRef> store;
+  store.Add("hello");
+
+  // Non-const values() returns MutableRange which uses Iterator
+  auto mutable_range = store.values();
+  auto it = mutable_range.begin();
+  // This calls Iterator::operator->
+  EXPECT_EQ(it->size(), 5);
+}
+
+TEST(ValueStore, IteratorConversion) {
+  ValueStore<IdentifierId, llvm::StringRef> store;
+  store.Add("hello");
+
+  auto mutable_range = store.values();
+  ValueStore<IdentifierId, llvm::StringRef>::Iterator it =
+      mutable_range.begin();
+  ValueStore<IdentifierId, llvm::StringRef>::ConstIterator cit = it;
+  EXPECT_EQ(*cit, "hello");
+}
+
 }  // namespace
 }  // namespace Carbon::Testing