For WARNS <= 3, change the clang warning flag -Wno-conversion to
-Wno-enum-conversion.  In earlier clang versions (before 3.2), the
latter did not exist, and suppressing enum conversion warnings was
really the goal of this warning suppression flag.

This should enable the same kind of warning again as was fixed by
r259072 ("incompatible integer to pointer conversion passing 'Elf_Addr'
(aka 'unsigned int') to parameter of type 'void *'"), and which was only
emitted by gcc.

Noticed by:	kib
MFC after:	3 days