[C++ for OpenCL] Lift restriction on reference members w local objects (#509)

* [C++ for OpenCL] Lift restriction on reference members for __local objects.

Co-authored-by: Marco Antognini <antognini.marco@gmail.com>
Co-authored-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

Author: 3 people

cxx4opencl/address_spaces.txt

@@ -342,11 +342,11 @@ created in the local address space have undefined state at the point of
 their declaration. Developers are free to define a special member function
 that can initialize local address space objects after their declaration. Any
 default values provided for the initialization of members in a class
-declaration are ignored when creating the local address space objects.
-Classes that are instantiated with local address space objects can not
-have references as their members. The same restriction applies recursively
-to the members and base classes. Destructors of local address space objects
-are not invoked automatically. They can be called manually if required.
+declaration are ignored when creating the local address space objects. Since
+the initialization is performed after the variable declaration, special handling
+is required for classes with data members that are references because their values
+can not be overwritten trivially. Destructors of local address space objects
+are not invoked automatically. They can be called manually.
 
 User defined constructors are not allowed to construct objects in `__constant`
 address space. Such objects can be initialized using literals and