[OpenACC] Private Clause on Compute Constructs (#90521)
The private clause is the first that takes a 'var-list', thus this has a
lot of additional work to enable the var-list type. A 'var' is a
traditional variable reference, subscript, member-expression, or
array-section, so checking of these is pretty minor.
Note: This ran into some issues with array-sections (aka sub-arrays)
that will be fixed in a follow-up patch.
