Prototype for coalescing equivalent specifics of the same generic. (#5314)

This is a working version for coalescing equivalent specifics of the
same generic, with *many* things to add and improve.

toolchain/lower/handle.cpp

@@ -288,6 +288,7 @@ auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
                 SemIR::VarStorage /* inst */) -> void {
   auto* type = context.GetType(SemIR::GetTypeOfInstInSpecific(
       context.sem_ir(), context.specific_id(), inst_id));
+  context.AddTypeToCurrentFingerprint(type);
 
   // Position the first alloca right before the start of the executable code in
   // the function.