[flang][openacc] Fix ambiguity in the self clause parsing

In the OpenACC specification, there are two different self clause. One for the
update directive with a var-list argument. This clause is a synonym of the host clause.
The second self clause is present for most of the compute construct and takes an optional
condition. To solve this ambiguity, the self clause for the update directive is directly
translated to a host clause during the parsing. The self clause in AccClause refers always
to the compute construct clause.

Reviewed By: kiranktp

Differential Revision: https://reviews.llvm.org/D90185