[NFC] Add a class ObjCProtocolQualifiers to wrap APIs for ObjC protocol list.

This is in preparation of adding a new type class ObjCTypeParamType that
can take protocol qualifiers. ObjCProtocolQualifiers will be shared between
ObjCObjectType and ObjCTypeParamType.
    
rdar://24619481
rdar://25060179

Differential Revision: http://reviews.llvm.org/D23078

llvm-svn: 279351