[OpenACC] Implement auto/seq/independent clause Sema for 'loop'

These three clauses are all quite trivial, as they take no parameters.
They are mutually exclusive, and 'seq' has some other exclusive

[OpenACC] Implement auto/seq/independent clause Sema for 'loop'

These three clauses are all quite trivial, as they take no parameters.
They are mutually exclusive, and 'seq' has some other exclusives that
are implemented here.

The ONE thing that isn't implemented is 2.9's restriction (line 2010):
'A loop associated with a 'loop' construct that does not have a 'seq'
clause must be written to meet all the following conditions'.

Future clauses will require similar work, so it'll be done as a
followup.

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[OpenACC] Implement Default clause for Compute Constructs (#88135)

As a followup to my previous commits, this is an implementation of a
single clause, in this case the 'default' clause. This implem

[OpenACC] Implement Default clause for Compute Constructs (#88135)

As a followup to my previous commits, this is an implementation of a
single clause, in this case the 'default' clause. This implements all
semantic analysis for it on compute clauses, and continues to leave it
rejected for all others (some as 'doesnt appertain', others as 'not
implemented' as appropriate).

This also implements and tests the TreeTransform as requested in the
previous patch.

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