Replace r313747, don't always warn on enums, rework testcases.

As Aaron Ballman has pointed out, that is not really correct.
So the key problem there is the invalidity of the testcase.

Revert r313747, and rework testcase in such a way, so these
details (platform-specific default enum sigdness) are
accounted for.

Also, add a C++-specific testcase.

llvm-svn: 313756