Diagnose uninitialized uses of a variable within its own initializer.

This is basically the same idea as the warning on uninitialized uses of
fields within an initializer list. As such, it is on by default and
under -Wuninitialized.

Original patch by Richard Trieu, with some massaging from me on the
wording and grouping of the diagnostics.

llvm-svn: 128376