[OpenACC] Remove 'loop' link to parent construct

After implementing 'loop', we determined that the link to its parent
only ever uses the type, not the construct itself. This patch removes
it, as it

[OpenACC] Remove 'loop' link to parent construct

After implementing 'loop', we determined that the link to its parent
only ever uses the type, not the construct itself. This patch removes
it, as it is both a waste and causes problems with serialization.

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[OpenACC] Implement `loop` restrictions on `for` loops. (#115370)

OpenACC restricts the contents of a 'for' loop affected by a 'loop'
construct without a 'seq'. The loop variable must be integer, p

[OpenACC] Implement `loop` restrictions on `for` loops. (#115370)

OpenACC restricts the contents of a 'for' loop affected by a 'loop'
construct without a 'seq'. The loop variable must be integer, pointer,
or random-access-iterator, it must monotonically increase/decrease, and
the trip count must be computable at runtime before the function.

This patch tries to implement some of these limitations to the best of
our ability, though it causes us to be perhaps overly restrictive at the
moment. I expect we'll revisit some of these rules/add additional
supported forms of loop-variable and 'monotonically increasing' here,
but the currently enforced rules are heavily inspired by the OMP
implementation here.

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[OpenACC] Loop construct basic Sema and AST work (#93742)

This patch implements the 'loop' construct AST, as well as the basic
appertainment rule. Additionally, it sets up the 'parent' compute
con

[OpenACC] Loop construct basic Sema and AST work (#93742)

This patch implements the 'loop' construct AST, as well as the basic
appertainment rule. Additionally, it sets up the 'parent' compute
construct, which is necessary for codegen/other diagnostics.

A 'loop' can apply to a for or range-for loop, otherwise it has no other
restrictions (though some of its clauses do).

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