Change ObjCIntefaceDecl::lookupMethod() to have optional 'followsSuper' argument.

This enables a micro-optimization in protocol conformance checking
to not examine the class hierarchy twice per method.

As part of this change, remove the default arguments from lookupInstanceMethod()
and lookupClassMethod().  It was becoming very redundant.  For clients
needing the default arguments, have them use the full API instead of
these convenience methods.
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.