[OpenACC] Implement 'reduction' for combined constructs.

Once again, this is a clause on a combined construct that does almost
exactly what the loop/compute construct version does, only with some sl

[OpenACC] Implement 'reduction' for combined constructs.

Once again, this is a clause on a combined construct that does almost
exactly what the loop/compute construct version does, only with some sl
ightly different evaluation rules/sema rules as it doesn't have to
consider the parent, just the 'combined' construct. The two sets of
rules for reduction on loop and compute are fine together, so this
ensures they are all enforced for this too.

The 'gangs' 'num_gangs' 'reduction' diagnostic (Dim>1) had to be applied
to num_gangs as well, as it previously wasn't permissible to get in this
situation, but we now can.

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[OpenACC] Implement 'reduction' sema for compute constructs (#92808)

'reduction' has a few restrictions over normal 'var-list' clauses:

1- On parallel, a num_gangs can only have 1 argument when c

[OpenACC] Implement 'reduction' sema for compute constructs (#92808)

'reduction' has a few restrictions over normal 'var-list' clauses:

1- On parallel, a num_gangs can only have 1 argument when combined with
reduction. These two aren't able to be combined on any other of the
compute constructs however.

2- The vars all must be 'numerical data types' types of some sort, or a
'composite of numerical data types'. A list of types is given in the
standard as a minimum, so we choose 'isScalar', which covers all of
these types and keeps types that are actually numeric. Other compilers
don't seem to implement the 'composite of numerical data types', though
we do.

3- Because of the above restrictions, member-of-composite is not
allowed, so any access via a memberexpr is disallowed. Array-element and
sub-arrays (aka array sections) are both permitted, so long as they meet
the requirements of #2.

This patch implements all of these for compute constructs.

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