Add tests for the uninitialized checks added in r128376. Also clean up

Add tests for the uninitialized checks added in r128376. Also clean up
and flesh out the existing uninitialized testing for field initializers.

The tests come from Richard's original patch, but I've cleaned them up
a bit and ordered them more naturally.

Also, I added a test for the most simple base case:
int x = x;

And it turns out we miss this one! =[ That and another bad FIXME on the
field initializer checking are left in the test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128394 91177308-0d34-0410-b5e6-96231b3b80d8