[analyzer] Make Malloc Checker track memory allocated by if_nameindex

The MallocChecker does currently not track the memory allocated by
if_nameindex. That memory is dynamically allocated and should be freed
by calling if_freenameindex. The attached patch teaches the checker
about these functions.

Memory allocated by if_nameindex is treated as a separate allocation
"family". That way the checker can verify it is freed by the correct
function.

A patch by Daniel Fahlgren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219025 91177308-0d34-0410-b5e6-96231b3b80d8