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.

llvm-svn: 195532