Merge from mainline.

tweak some pointer sema checking stuff (which was added to implement PR4175) to
avoid emitting a warning on "someptr > 0".  This is obviously questionable (they
could use != instead) but is reasonable, and the warning "ordered comparison
between pointer and integer" didn't make a ton of sense because 0 is a valid
null pointer constant.

Just silence the warning in this case, it is unlikely to indicate a bug.

llvm-svn: 79783