[flang][openacc] Accept !$acc end loop

Some compilers accept `!$acc end loop` associated with an `!$acc loop`
directive. This patch updates the acc loop parser to accept it as well.
The parser is also updated to be stricter on the following statement
to match the OpenACC combined construct parser.
The rewrite canonicalization is not a rewrite anymore and the naming
will be updated in a follow up patch for the Loop and Combined constructs.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D159015