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/lex/tokenized_buffer_benchmark.cpp

@@ -11,6 +11,7 @@
 #include "common/check.h"
 #include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/StringExtras.h"
+#include "toolchain/base/value_store.h"
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/diagnostics/null_diagnostics.h"
 #include "toolchain/lex/token_kind.h"
@@ -374,14 +375,14 @@ class LexerBenchHelper {
 
   auto Lex() -> TokenizedBuffer {
     DiagnosticConsumer& consumer = NullDiagnosticConsumer();
-    return TokenizedBuffer::Lex(source_, consumer);
+    return TokenizedBuffer::Lex(value_stores_, source_, consumer);
   }
 
   auto DiagnoseErrors() -> std::string {
     std::string result;
     llvm::raw_string_ostream out(result);
     StreamDiagnosticConsumer consumer(out);
-    auto buffer = TokenizedBuffer::Lex(source_, consumer);
+    auto buffer = TokenizedBuffer::Lex(value_stores_, source_, consumer);
     consumer.Flush();
     CARBON_CHECK(buffer.has_errors())
         << "Asked to diagnose errors but none found!";
@@ -398,6 +399,7 @@ class LexerBenchHelper {
         fs_, filename_, ConsoleDiagnosticConsumer()));
   }
 
+  SharedValueStores value_stores_;
   llvm::vfs::InMemoryFileSystem fs_;
   std::string filename_ = "test.carbon";
   SourceBuffer source_;