Add a check to the FindBadConstructs.cpp clang plugin for bad enum last values.

One common coding patern is to define an enum as in:
  enum Color { RED, GREEN, BLUE, COLOR_LAST=BLUE };
but this is fragile when someone adds a new constant and forgets
to update the COLOR_LAST constant.

This change looks for enums that have a xxx_LAST or xxxLast member,
and warns if there are any higher-valued constants present.

Review URL: https://codereview.chromium.org/150943005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251894 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251894