[flang][acc] Lower zero modifier for Copyout clause (#166660)

From the OpenACC 3.4 Specification:
```
1924 2.7.9 copyout clause
1925 The copyout clause may appear on structured data and compute constructs, on declare di
1926 rectives, and on exit data directives. The clause may optionally have a zero modifier if the
1927 copyout clause appears on a structured data or compute construct.
1928 Only the following modifiers may appear in the optional modifier-list: always, alwaysin or zero.
1929 Additionally, on structured data and compute constructs capture modifier may appear
```
`readonly` is not a legal modifier for the `copyout` clause. The call to
`genDataOperandOperationsWithModifier` should be checking the parsed
modifier for the `copyout` clause against the `Zero` modifier.

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