c++: constexpr and -fno-elide-constructors [PR101072]

c++: constexpr and -fno-elide-constructors [PR101072]

We've been trying for a while to avoid TARGET_EXPRs in template code, but
there were still a few that snuck through, and the one in this case broke
the code that tried to handle it.  Fixed by using IMPLICIT_CONV_EXPR, as we
have done elsewhere.

I also noticed that finish_compound_literal was assuming that all T{init}
were for aggregate T, and we got a few more TARGET_EXPRs from that.  Fixed
by only messing with TARGET_EXPR if we actually have an aggregate init.

	PR c++/101072

gcc/cp/ChangeLog:

	* cp-tree.h (build_implicit_conv_flags): Declare.
	* call.cc (build_implicit_conv_flags): Split out from...
	(perform_implicit_conversion_flags): ...here.
	* decl.cc (check_initializer): Use it.
	* pt.cc (tsubst_copy_and_build): Remove TARGET_EXPR handling.
	* semantics.cc (finish_compound_literal): Don't treat
	scalar values like CONSTRUCTORs.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty14a.C: New test.