Introduce a new warning, -Wtop-level-comparison. This warning is

a complement to the warnings we provide in condition expressions. Much
like we warn on conditions such as:

  int x, y;
  ...
  if (x = y) ... // Almost always a typo of '=='

This warning applies the complementary logic to "top-level" statements,
or statements whose value is not consumed or used in some way:

  int x, y;
  ...
  x == y; // Almost always a type for '='

We also mirror the '!=' vs. '|=' logic.

The warning is designed to fire even for overloaded operators for two reasons:

1) Especially in the presence of widespread templates that assume
   operator== and operator!= perform the expected comparison operations,
   it seems unreasonable to suppress warnings on the offchance that
   a user has written a class that abuses these operators, embedding
   side-effects or other magic within them.
2) There is a trivial source modification to silence the warning for
   truly exceptional cases:

     (void)(x == y); // No warning

A (greatly reduced) form of this warning has already caught a number of
bugs in our codebase, so there is precedent for it actually firing. That
said, its currently off by default, but enabled under -Wall.

There are several fixmes left here that I'm working on in follow-up
patches, including de-duplicating warnings from -Wunused, sharing code
with -Wunused's implementation (and creating a nice place to hook
diagnostics on "top-level" statements), and handling cases where a proxy
object with a bool conversion is returned, hiding the operation in the
cleanup AST nodes.

Suggestions for any of this code more than welcome. Also, I'd really
love suggestions for better naming than "top-level".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137819 91177308-0d34-0410-b5e6-96231b3b80d8

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