[OpenMP] fix seg-faults printing diagnostics with invalid ordered(n) values

When the value specified for n in ordered(n) is larger than the number of loops a segmentation fault can occur in one of two ways when attempting to print out a diagnostic for an associated depend(sink : vec):
1) The iteration vector vec contains less than n items
2) The iteration vector vec contains a variable that is not a loop control variable
This patch addresses both of these issues.

Differential Revision: https://reviews.llvm.org/D38049

llvm-svn: 313675