Modify the uninitialized field visitor to detect uninitialized use across the
fields in the class.  This allows a better checking of member intiailizers and
in class initializers in regards to initialization ordering.

For instance, this code will now produce warnings:

class A {
  int x;
  int y;
  A() : x(y) {}  // y is initialized after x, warn here
  A(int): y(x) {} // default initialization of leaves x uninitialized, warn here
};

Several test cases were updated with -Wno-uninitialized to silence this warning.