Special-casing these as MemberRefExprs created an asymmetry
where unbound archetype instance methods (<T : P> T.f) could
not be represented. Treating class and protocol methods
uniformly also eliminates a handful of special cases around
MemberRefExpr.
SILGen's RValue and call emission peepholes now have to know
about DeclRefExprs that point to protocol methods.
Finally, generalize the diagnostic for partially applied
mutating methods to any partially applied function with an
inout parameter, since this is not supported.
Fixes <rdar://problem/20564672>.
Swift SVN r29298
