Begin refactoring the uninitialized warning code that I uglied up. This

extracts a function to handle the emission of the diagnostic separately
from the walking over the set of uninitialized uses.

Also updates the naming used within this extracted function to be a bit
more consistent with the rest of Clang's naming patterns.

The next step will be breaking this apart so that we can go through
different functions rather than tracking so many boolean variables.

llvm-svn: 128898