[OpenCL] Map default address space to alloca address space

[OpenCL] Map default address space to alloca address space

For OpenCL, the private address space qualifier is 0 in AST. Before this change, 0 address space qualifier
is always mapped to target address space 0. As now target private address space is specified by
alloca address space in data layout, address space qualifier 0 needs to be mapped to alloca addr space specified by the data layout.

This change has no impact on targets whose alloca addr space is 0.

With contributions from Matt Arsenault, Tony Tye and Wen-Heng (Jack) Chung

Differential Revision: https://reviews.llvm.org/D31404

llvm-svn: 299965