c++: C++17 and decltype of multi-operator expression [PR95675]

c++: C++17 and decltype of multi-operator expression [PR95675]

A call that is the immediate operand of decltype has special semantics: no
temporary is produced, so it's OK for the return type to be e.g. incomplete.
But we were treating (e | f) the same way, which confused overload
resolution when we then tried to evaluate ... | g.

Fixed by making build_temp do what its name says, and force the C++17
temporary materialization conversion.

gcc/cp/ChangeLog:

	PR c++/95675
	* call.c (build_temp): Wrap a CALL_EXPR in a TARGET_EXPR
	if it didn't get one before.

gcc/testsuite/ChangeLog:

	PR c++/95675
	* g++.dg/cpp0x/decltype-call5.C: New test.
	* g++.dg/cpp0x/decltype-call6.C: New test.