[OPENMP 4.1] Initial support for extended 'ordered' clause.

OpenMP 4.1 introduces optional argument '(n)' for 'ordered' clause, where 'n' is a number of loops that immediately follow the directive.
'n' must be constant positive integer expressions and it must be less or equal than the number of the loops in the resulting loop nest.
Patch adds parsing and semantic analysis for this optional argument.

llvm-svn: 243635