[analyzer] Tweak MallocSizeOfChecker to not warn when using sizeof(void*) to...

[analyzer] Tweak MallocSizeOfChecker to not warn when using sizeof(void*) to allocate a bunch of any pointer type.

This suppresses a common false positive when analyzing libc++.

Along the way, introduce some tests to show this checker actually
works with C++ static_cast<>.

llvm-svn: 220160