ScopGraphPrinter: Only show functions that contain at least one scop

When running 'clang -O3 -mllvm -polly -mllvm -polly-show' we now only show the
CFGs of functions with at least one detected scop. For larger files/projects
this reduces the number of graphs printed significantly and is likely what
developers want to see. The new option -polly-view-all enforces all graphs to be
printed and the exiting option -poll-view-only limites the graph printing to
functions that match a certain pattern.

This patch requires https://llvm.org/svn/llvm-project/llvm/trunk@255889 (and
vice versa) to compile correctly.

llvm-svn: 255891