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/eval.cpp

@@ -1717,7 +1717,7 @@ static auto MakeConstantForCall(EvalContext& eval_context,
     // Calls to builtins might be constant.
     builtin_kind = eval_context.functions()
                        .Get(callee_function.function_id)
-                       .builtin_function_kind;
+                       .builtin_function_kind();
     if (builtin_kind == SemIR::BuiltinFunctionKind::None) {
       // TODO: Eventually we'll want to treat some kinds of non-builtin
       // functions as producing constants.