[clang-tidy] Add a checker for zero-length memset.

If there's memset(x, y, 0) in the code it's most likely a mistake. The
checker suggests a fix-it to swap 'y' and '0'.

I think this has the potential to be promoted into a general clang warning
after some testing in clang-tidy.

Differential Revision: http://reviews.llvm.org/D4535