c++: Quash bogus -Wredundant-move warning

c++: Quash bogus -Wredundant-move warning

This patch fixes a pretty stoopid thinko.  When I added code to warn
about pessimizing std::move in initializations like

  T t{std::move(T())};

I also added code to unwrap the expression from { }.  But when we have

  return {std::move(t)};

we cannot warn about a redundant std::move because the implicit move
wouldn't happen for "return {t};" because the expression isn't just
a name.  However, we still want to warn about

  return {std::move(T())};

so let's not disable the -Wpessimizing-move warning.  Tests added for
both cases.

gcc/cp/ChangeLog:

	* typeck.cc (maybe_warn_pessimizing_move): Don't warn about
	redundant std::move when the expression was wrapped in { }.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/Wpessimizing-move10.C: New test.
	* g++.dg/cpp0x/Wredundant-move12.C: New test.