CodeGen: Add scop-parameters to the OpenMP context

In addition to the arrays and clast variables a SCoP statement may also refer to
values defined before the SCoP or to function arguments. Detect these values and
add them to the set of values passed to the function generated for OpenMP
parallel execution of a clast.

Committed with additional test cases and some refactoring.

Contributed by:  Armin Groesslinger  <armin.groesslinger@uni-passau.de>

llvm-svn: 167214