Add info to ObjCPropertyRefExpr to indicate whether the dot syntax property

reference is going to message the setter, the getter, or both.

Having this info on the ObjCPropertyRefExpr node makes it easier for AST
clients (like libclang) to reason about the meaning of the property reference.

[AST/Sema]
-Use 2 bits (with a PointerIntPair) in ObjCPropertyRefExpr to record the above info
-Have ObjCPropertyOpBuilder set the info appropriately.

[libclang]
-When there is an implicit property reference (property syntax using methods)
have clang_getCursorReferenced return a cursor for the method. If the property
reference is going to result in messaging both the getter and the setter choose
to return a cursor for the setter because it is less obvious from source inspection
that the setter is getting called.

The general idea has the seal of approval by John.

rdar://11151621

llvm-svn: 153709