Handle 'instancetype' in ParseDeclarationSpecifiers.

...instead of as a special case in ParseObjCTypeName with lots of
duplicated logic. Besides being a nice refactoring, this also allows
"- (instancetype __nonnull)self" in addition to "- (nonnull instancetype)self".

rdar://problem/19924646

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240188 91177308-0d34-0410-b5e6-96231b3b80d8