re PR c++/10245 (?: operator requires public copy constructor of return type)

PR c++/10245
        * cvt.c (force_rvalue): New fn.
        * call.c (build_conditional_expr): Use it.
        * cp-tree.h: Declare it.

[[Split portion of a mixed commit.]]

From-SVN: r65006.2

gcc/testsuite/g++.dg/conversion/cond2.C

@@ -0,0 +1,15 @@
+// PR c++/10245
+
+struct X {};
+    
+struct Y {
+  Y ();
+  operator X () const;
+private:
+  Y (const Y &);
+};
+
+Y y;
+void foo() {
+  X x = (1 ? Y() : Y());
+}