[gomp4] Add front end support for the if_present clause with the update directive

[Cesar Philippidis]

This patch make the c, c++ and fortran FEs aware of the new OpenACC 2.5
if_present clause for the update directive. The ME and runtime support
will come in a separate followup patch.

Thomas, for some reason I'm seeing a couple of new UNRESOLVED tests for
update-1.C. The c++ tests running with goacc.exp are built with
-fopenacc, but for some reason the tests in g++.dg/goacc/ are still ran
without -fopenacc for g++.dg/dg.exp. Maybe there's something wrong with
g++.dg/goacc/goacc.exp handling of .C files?

This patch has been committed to gomp-4_0-branch.

Cesar
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