[C++ for OpenCL] Added __remove_address_space feature (#647)

Add documentation of  __remove_address_space functionality
in C++ for OpenCL 2021.

Author: Topotuna

cxx4opencl/address_spaces.txt

@@ -551,3 +551,31 @@ defdefptr = addrspace_cast<int * *>(cnstdefptr); // illegal.
 defdefptr = reinterpret_cast<int * *>(locdefptr); // legal.
 defdefptr = reinterpret_cast<int * *>(cnstdefptr); // legal.
 ------------
+
+[[remove-addrspace]]
+==== Address space removal
+
+`+template<class T> struct __remove_address_space;+`
+
+{cpp} for OpenCL 2021 supports utility `+__remove_address_space+` which allows to
+derive types that have address space qualifiers removed. Its effect is analogous
+to `+__remove_const+` and other similar traits in {cpp}17 `[meta.trans]`. The
+utility only removes an address space qualifier from a given type, therefore, other
+type qualifiers such as `+const+` or `+volatile+` remain unchanged.
+
+[source,cpp]
+----------
+template<typename T>
+void foo(T *par) {
+  T var1; // error: function scope variable cannot be declared in global
+          // address space
+  __private T var2; // error: conflicting address space qualifiers are provided
+                    // between types '__private T' and '__global int'
+  __private __remove_address_space<T>::type var3; // type of var3 is __private int
+}
+
+void bar() {
+  __global int* ptr;
+  foo(ptr);
+}
+----------