Introduce a placeholder type for "pseudo object"

Introduce a placeholder type for "pseudo object"
expressions: expressions which refer to a logical rather
than a physical l-value, where the logical object is
actually accessed via custom getter/setter code.
A subsequent patch will generalize the AST for these
so that arbitrary "implementing" sub-expressions can
be provided.

Right now the only client is ObjC properties, but
this should be generalizable to similar language
features, e.g. Managed C++'s __property methods.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142914 91177308-0d34-0410-b5e6-96231b3b80d8