Problem: The intrinsics openmp frontend must create directive nests to
codegen from innermost to outermost directive for avoiding value
renaming. Previously, it created a list of directives which lacked
topological ordering and could break with deep nests.

This commit updates the frontend to correctly generate directive nests
as trees in topological order. For codegen, it creates again a
topologically ordered list of directives processed from innermost to
outermost.