[OpenCL] Use generic addr space for lambda call operator

Since lambdas are represented by callable objects, we add
generic addr space for implicit object parameter in call
operator.

Any lambda variable declared in __constant addr space
(which is not convertible to generic) fails to compile with
a diagnostic. To support constant addr space we need to
add a way to qualify the lambda call operators.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69938