Mangle the signature decl when mangling a thunk. (#7177)

Fixes mangling collisions when two thunks with the same name (eg, `Op`)
are created in the same context, which in turn would lead to LLVM
verifier failures and miscompiles.

To support this, add a new value store to track a little more
information about thunks beyond what's in the `Function`.

toolchain/check/thunk.cpp

@@ -29,6 +29,7 @@
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
 #include "toolchain/sem_ir/pattern.h"
+#include "toolchain/sem_ir/thunk.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
@@ -489,12 +490,17 @@ auto BuildThunk(Context& context, SemIR::FunctionId signature_id,
   // We can't use the function directly. Build a thunk.
   // TODO: Check for and diagnose obvious reasons why this will fail, such as
   // arity mismatch, before trying to build the thunk.
-  auto [function_id, thunk_id] =
+  auto [function_id, thunk_inst_id] =
       CloneFunctionDecl(context, SemIR::LocId(callee_id), signature_id,
                         signature_specific_id, callee.function_id);
 
+  auto thunk_id =
+      context.sem_ir().thunks().Add({.callee_id = callee_id,
+                                     .signature_id = signature_id,
+                                     .specific_id = signature_specific_id});
+
   // Track that this function is a thunk.
-  context.functions().Get(function_id).SetThunk(callee_id);
+  context.functions().Get(function_id).SetThunk(thunk_id);
 
   if (defer_definition) {
     // Register the thunk to be defined when we reach the end of the enclosing
@@ -504,16 +510,16 @@ auto BuildThunk(Context& context, SemIR::FunctionId signature_id,
         context, {
                      .signature_id = signature_id,
                      .function_id = function_id,
-                     .decl_id = thunk_id,
+                     .decl_id = thunk_inst_id,
                      .callee_id = callee_id,
                  });
   } else {
-    BuildThunkDefinition(context, signature_id, function_id, thunk_id,
+    BuildThunkDefinition(context, signature_id, function_id, thunk_inst_id,
                          callee_id);
     context.scope_stack().Pop();
   }
 
-  return thunk_id;
+  return thunk_inst_id;
 }
 
 auto BuildDestroyThunk(Context& context, SemIR::LocId loc_id,
@@ -528,7 +534,7 @@ auto BuildDestroyThunk(Context& context, SemIR::LocId loc_id,
                                  .self_type_id = class_info.self_type_id});
 
   auto& thunk_function = context.functions().Get(thunk_function_id);
-  thunk_function.SetThunk(SemIR::InstId::None);
+  thunk_function.SetThunk(SemIR::ThunkId::None);
 
   context.scope_stack().PushForDeclName();
   StartFunctionDefinition(context, thunk_inst_id, thunk_function_id);