Class Property: change PropertyMap to include isClassProperty.

PropertyMap used to map IdentifierInfo (name of the property) to
ObjcPropertyDecl *. Now that a class property can have the same name as
an instance property, we change PropertyMap to map a pair <IdentifierInfo *,
unsigned> to ObjcPropertyDecl *.

Also update a few places from iterating over instance_properties to
iterating over all properties.

rdar://23891898


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259119 91177308-0d34-0410-b5e6-96231b3b80d8