[ODRHash] Fix null pointer dereference for ObjC selectors with empty slots.

`Selector::getIdentifierInfoForSlot` returns NULL if a slot has no
corresponding identifier. Add a boolean to the hash and a NULL check.

rdar://problem/51615164

Reviewers: rtrieu

Reviewed By: rtrieu

Subscribers: dexonsmith, cfe-commits, jkorous

Differential Revision: https://reviews.llvm.org/D63789

llvm-svn: 364664