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/parse/parse_fuzzer.cpp

@@ -6,6 +6,7 @@
 #include <cstring>
 
 #include "llvm/ADT/StringRef.h"
+#include "toolchain/base/value_store.h"
 #include "toolchain/diagnostics/null_diagnostics.h"
 #include "toolchain/lex/tokenized_buffer.h"
 #include "toolchain/parse/tree.h"
@@ -31,7 +32,9 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
       SourceBuffer::CreateFromFile(fs, TestFileName, NullDiagnosticConsumer());
 
   // Lex the input.
-  auto tokens = Lex::TokenizedBuffer::Lex(*source, NullDiagnosticConsumer());
+  SharedValueStores value_stores;
+  auto tokens = Lex::TokenizedBuffer::Lex(value_stores, *source,
+                                          NullDiagnosticConsumer());
   if (tokens.has_errors()) {
     return 0;
   }