[flang][OpenMP] Don't allow DO CONCURRENT inside of a loop nest (#144506)

I don't think DO CONCURRENT fits the definition of a Canonical Loop Nest
(OpenMP 6.0 section 6.4.1).
It is however explicitly allowed for the LOOP construct (6.0 section
13.8).

There's some obscure language in OpenMP 6.0 for the LOOP construct:

> If the collapsed loop is a DO CONCURRENT loop, neither the
> data-sharing attribute clauses nor the collapse clause may be
specified.

From the surrounding context, I think "collapsed loop" just means the
loop that the LOOP construct applies to. So I will interpret this to
mean that DO CONCURRENT can only be used with the LOOP construct if it
does not contain the COLLAPSE clause.

This also fixes a bug where the associated clause was never cleared
after it was set.

Fixes #144178

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