[asan] implement more strict checking for memset/etc parameters. Instead of...

[asan] implement more strict checking for memset/etc parameters. Instead of checking the first and the last byte, we check the entire shadow region. This costs ~10 slowdown for the instrumented functions. Motivated by a nasty memset-buffer-overflow-by-140-bytes in chrome which was reported as a use-after-free or not at all

llvm-svn: 171198