[OPENMP] Update the diagnosis message for canonical loop form, by Chi

Chun Chen.

The previous patch (https://reviews.llvm.org/D54441) support the
relational-op != very well for openmp canonical loop form, however,
it didn't update the diagnosis message. So this patch is simply
update the diagnosis message by adding !=, update the test
related to it, and update the section number for canonical loop
form for OpenMP 5.0 in comment.

Differential Revision: https://reviews.llvm.org/D66559

llvm-svn: 371631