[OpenCL] Handle taking an address of block captures.

Block captures can have different physical locations
in memory segments depending on the use case (as a function
call or as a kernel enqueue) and in different vendor
implementations.

Therefore it's unclear how to add address space to capture
addresses uniformly. Currently it has been decided to disallow
taking addresses of captured variables until further
clarifications in the spec.

Differential Revision: https://reviews.llvm.org/D36410

llvm-svn: 312728