CodeGen: Fix address space of global variable

Certain targets (e.g. amdgcn) require global variable to stay in global or constant address
space. In C or C++ global variables are emitted in the default (generic) address space.
This patch introduces virtual functions TargetCodeGenInfo::getGlobalVarAddressSpace
and TargetInfo::getConstantAddressSpace to handle this in a general approach.

It only affects IR generated for amdgcn target.

Differential Revision: https://reviews.llvm.org/D33842

llvm-svn: 307470