c++: Fix wrong conversion error with non-viable overload [PR94124]

c++: Fix wrong conversion error with non-viable overload [PR94124]

This is a bad interaction between sharing a constructor for an array
and stripping its trailing zero-initializers.  Here we reuse a ctor
and then strip its 0s.  This breaks overload resolution in this test:
D can be initialized from {} but not from {0}, so if we truncate the
constructor not to include the zero, the F(D) overload becomes valid
and then we get the ambiguous conversion error.

	PR c++/94124 - wrong conversion error with non-viable overload.
	* decl.c (reshape_init_array_1): Unshare a constructor if we
	stripped trailing zero-initializers.

	* g++.dg/cpp0x/initlist-overload1.C: New test.