[NFC] Bump ObjCOrBuiltinIDBits to 15

We're now hitting this because we're at the limit for number of builtins:
  clang/lib/Basic/IdentifierTable.cpp:39:1: error: static_assert failed due to requirement '2 * LargestBuiltinID < (2 << (ObjCOrBuiltinIDBits - 1))' "Insufficient ObjCOrBuiltinID Bits"
  static_assert(2 * LargestBuiltinID < (2 << (ObjCOrBuiltinIDBits - 1)),
  ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bump it to 15 so whoever adds a builtin next (as I am, or as anyone else might) doesn't merge conflict over each other.