Implement the Objective-C 'instancetype' type, which is an alias of

Implement the Objective-C 'instancetype' type, which is an alias of
'id' that can be used (only!) via a contextual keyword as the result
type of an Objective-C message send. 'instancetype' then gives the
method a related result type, which we have already been inferring for
a variety of methods (new, alloc, init, self, retain). Addresses
<rdar://problem/9267640>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139275 91177308-0d34-0410-b5e6-96231b3b80d8