PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.

PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.

After r269667 which introduced joust_maybe_elide_copy, in C++17 we can elide
a constructor if it uses a conversion function that returns a prvalue, and
use the conversion function in its stead.

This eliding means that if we have a candidate that previously didn't have
->second_conv, it can have it after the elision.  This confused the
-Wconversion warning because it was assuming that if cand1->second_conv is
non-null, so is cand2->second_conv.  Here cand1->second_conv was non-null
but cand2->second_conv remained null, so it crashed in compare_ics.

I checked with clang that both compilers call A::operator B() in C++17 and
B::B(A const &) otherwise.

	* call.c (joust): Don't attempt to warn if ->second_conv is null.

	* g++.dg/cpp0x/overload-conv-4.C: New test.

From-SVN: r277593