Add well-known identifier caching (#6486)

I'm doing this because I figured it'd be an incremental improvement for
all the operator lookups that we do. Even to the extent that we've
discussed witness caching, I think it'll still apply. It does add one
more step to adding new interfaces (before, you'd just write the string,
now you add it to the def file and reference it).

I'll claim it makes GetClangOperatorKind a lot friendlier to read/edit,
nevermind removing the string comparisons. :)

toolchain/check/context.cpp

@@ -38,7 +38,8 @@ Context::Context(DiagnosticEmitterBase* emitter,
       global_init_(this),
       region_stack_([this](SemIR::LocId loc_id, std::string label) {
         TODO(loc_id, label);
-      }) {
+      }),
+      core_identifiers_(&identifiers()) {
   // Prepare fields which relate to the number of IRs available for import.
   import_irs().Reserve(imported_ir_count);
   import_ir_constant_values_.reserve(imported_ir_count);