Issue a warning when a variable is used to initialize itself

[ehaas]

.uninitialized_in_own_init should be issued if a variable is used in its own initializer (there are some caveats, see the lines marked with -Wuninitialized in the initializers.c test case)

[Vexu]

The same mechanism should also be able to detect use of uninitialized variables in general (in the limited scope of per function static analysis).