Rename Carbon::StringLiteralId -> Carbon::StringLiteralValueId. (#3522)

We have `StringLiteral`s in multiple other `Carbon` sub-namespaces.
Rename to a more specific name to avoid collisions.

We should likely also rename `Carbon::IntId` -> `Carbon::IntValueId` and
`Carbon::RealId` -> `Carbon::RealValueId`, but this collision is
prioritized because it was blocking work on typed parse nodes which
introduces a `Carbon::Parse::StringLiteralId`.

toolchain/check/check.cpp

@@ -225,7 +225,7 @@ static auto GetImportKey(UnitInfo& unit_info, IdentifierId file_package_id,
                                    : "";
   llvm::StringRef library_name =
       names.library_id.is_valid()
-          ? stores->string_literals().Get(names.library_id)
+          ? stores->string_literal_values().Get(names.library_id)
           : "";
   return {package_name, library_name};
 }