First cut at warning about expressions whose results are ignored. For example,

this produces:

warn.c:4:3: warning: expression result unused
  X == Y;
  ^~~~~~
warn.c:5:3: warning: expression result unused
  (void)X;
  ^~~~~~~
warn.c:11:3: warning: expression result unused
  A == foo(1, 2);
  ^~~~~~~~~~~~~~
warn.c:13:3: warning: expression result unused
  foo(1,2)+foo(4,3);
  ^~~~~~~~~~~~~~~~~

llvm-svn: 39682