[flang] Allow compiler directives for collapsed loops in OpenACC

If one tries to compile the attached test case code with flan,
the one will get an internal compiler error on `CHECK(level == 0)`
at the end of `PrivatizeAssociatedLoopIndex` function.
Other compilers (gfortran and nvfortran) build this code just fine.
This change fixes the ICE.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D132846