[flang][openacc] Handle optional end directive in combined construct

OpenACC combined construct can have an optional end directive. This patch handle this
case in the parsing/unparsing with a canonicalization step. Unlike OmpEndLoopDirective,
this doesn't need a special treatment in the pre-fir tree as there is no clause attached to
a AccEndCombinedDirective.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D84481