[analyzer] Allow IvarInvalidation checker to suppress warnings via
assertions.

To ensure that custom assertions/conditional would also be supported,
just check if the ivar that needs to be invalidated or set to nil is
compared against 0.

Unfortunately, this will not work for code containing 'assert(IvarName)'

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172147 91177308-0d34-0410-b5e6-96231b3b80d8