During cross field uninitialized checking, when processing an assignment,
don't mark the field as initialized until the next initializer instead of
instantly.  Since this checker is AST based, statements are processed in tree
order instead of following code flow.  This can result in different warnings
from just reordering the code.  Also changed to use one checker per constructor
instead of creating a new checker per field.

class T {
  int x, y;

  // Already warns
  T(bool b) : x(!b ? (1 + y) : (y = 5)) {}

  // New warning added here, previously (1 + y) comes after (y = 5) in the AST
  // preventing the warning.
  T(bool b) : x(b ? (y = 5) : (1 + y)) {}

};