Warn on mismatched parentheses in memcmp and friends.

Thisadds a new warning that warns on code like this:

  if (memcmp(a, b, sizeof(a) != 0))

The warning looks like:

test4.cc:5:30: warning: size argument in 'memcmp' call is a comparison [-Wmemsize-comparison]
  if (memcmp(a, b, sizeof(a) != 0))
                   ~~~~~~~~~~^~~~
test4.cc:5:7: note: did you mean to compare the result of 'memcmp' instead?
  if (memcmp(a, b, sizeof(a) != 0))
      ^                          ~
                            )
test4.cc:5:20: note: explicitly cast the argument to size_t to silence this warning
  if (memcmp(a, b, sizeof(a) != 0))
                   ^
                   (size_t)(     )
1 warning generated.

This found 2 bugs in chromium and has 0 false positives on both chromium and
llvm.

The idea of triggering this warning on a binop in the size argument is due to
rnk.

llvm-svn: 198063