[OpenCL] Prevent generation of globals in non-constant AS for OpenCL.

Avoid using shortcut for const qualified non-constant address space
aggregate variables while generating them on the stack such that
the alloca object is used instead of a global variable containing
initializer.

Review: https://reviews.llvm.org/D27109



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288163 91177308-0d34-0410-b5e6-96231b3b80d8