When making a direct call to a thunk, inline the call in SemIR. (#5642)

This preserves the constant values of the arguments to the thunk, which
is important if the thunk requires conversion of an `IntLiteral` to some
other type. This should become unnecessary once we have form support,
but avoiding the indirection through a thunk function seems valuable
even once that support is in place.

To support this, track whether a function is a thunk on the Function
object, and if so, what the callee of the thunk is. This information is
also included in formatted SemIR when dumping the thunk.

toolchain/check/handle_function.cpp

@@ -727,7 +727,7 @@ auto HandleParseNode(Context& context,
 
     auto& function = context.functions().Get(function_id);
     if (IsValidBuiltinDeclaration(context, function, builtin_kind)) {
-      function.builtin_function_kind = builtin_kind;
+      function.SetBuiltinFunction(builtin_kind);
       // Build an empty generic definition if this is a generic builtin.
       StartGenericDefinition(context, function.generic_id);
       FinishGenericDefinition(context, function.generic_id);