Use the correct address space when emitting the ctor function list

This patch modifies clang so that, if compiling for a target that
explicitly specifies a nonzero program memory address space, the
constructor list global will have the same address space as the
functions it contains.

AVR is the only in-tree backend which has a nonzero program memory
address space.

Without this, the IR verifier would always fail if a constructor
was used on a Harvard architecture backend.

This has no functional change to any in-tree backends except AVR.

llvm-svn: 346520