GPGPU: Generate an AST for the GPU-mapped schedule

GPGPU: Generate an AST for the GPU-mapped schedule

For this we need to provide an explicit list of statements as they occur in
the polly::Scop to ppcg.

We also setup basic AST printing facilities to facilitate debugging. To allow
code reuse some (minor) changes in ppcg are have been necessary.

llvm-svn: 275436