Add back experimental attribute objc_suppress_protocol_methods (slightly renamed).

This is still an experimental attribute, but I wanted it in tree
for review.  It may still get yanked.

This attribute can only be applied to a class @interface, not
a class extension or category.  It does not change the type
system rules for Objective-C, but rather the implementation checking
for Objective-C classes that explicitly conform to a protocol.
During protocol conformance checking, clang recursively searches
up the class hierarchy for the set of methods that compose
a protocol.  This attribute will cause the compiler to not consider
the methods contributed by a super class, its categories, and those
from its ancestor classes.  Thus this attribute is used to force
subclasses to redeclare (and hopefully re-implement) methods if
they decide to explicitly conform to a protocol where some of those
methods may be provided by a super class.

This attribute intentionally leaves out properties, which are associated
with state.  This attribute only considers methods (at least right now)
that are non-property accessors.  These represent methods that "do something"
as dictated by the protocol.  This may be further refined, and this
should be considered a WIP until documentation gets written or this
gets removed.

llvm-svn: 195533