[analyzer] Extend UnixAPIChecker open() checks to handle openat().

The openat() API has similar constraints to the open() API -- it just takes
an extra parameter.

rdar://problem/29526458

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290005 91177308-0d34-0410-b5e6-96231b3b80d8