In this PR, instantiating the static thunk returned from the generic lambda
conversion function template was using normal overload resolution, which
meant calling an extra constructor when forwarding its argument.  Fixed by
special-casing thunk calls significantly more.

	* lambda.c (maybe_add_lambda_conv_op): Use a template-id in the
	call.  Only forward parms for decltype.
	* pt.c (tsubst_copy_and_build) [CALL_EXPR]: Handle CALL_FROM_THUNK_P
	specially.
	* typeck.c (check_return_expr): Don't mess with a thunk call.

From-SVN: r268377