c++: fix missing lifetime extension [PR119383]

c++: fix missing lifetime extension [PR119383]

Since r15-8011 cp_build_indirect_ref_1 won't do the *&TARGET_EXPR ->
TARGET_EXPR folding not to change its value category.  That fix seems
correct but it made us stop extending the lifetime in this testcase,
causing a wrong-code issue -- extend_ref_init_temps_1 did not see
through the extra *& because it doesn't use a tree walk.

This patch reverts r15-8011 and instead handles the problem in
build_over_call by calling force_lvalue in the is_really_empty_class
case as well as in the general case.

	PR c++/119383

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Use force_lvalue to ensure op= returns
	an lvalue.
	* cp-tree.h (force_lvalue): Declare.
	* cvt.cc (force_lvalue): New.
	* typeck.cc (cp_build_indirect_ref_1): Revert r15-8011.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/temp-extend3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>