[flang][Semantics][OpenMP] don't reduce variables in namelist (#110671)

This is allowed by the OpenMP and F23 standards. But variables in a
namelist are not allowed in OpenMP privatisation. I suspe

[flang][Semantics][OpenMP] don't reduce variables in namelist (#110671)

This is allowed by the OpenMP and F23 standards. But variables in a
namelist are not allowed in OpenMP privatisation. I suspect this was an
oversight.

If we allow this we run into problems masking the original symbol with
the symbol for the reduction variable when the variable is accessed via
a namelist initialised as a global variable. See #101907. One solution
for this would be to force the namelist to always be initilized inside
of the block in which it is used (therefore using the correct mapping
for the reduction variable), but this could make some production
applications slow.

I tentatively think it is probably better to disallow a (perhaps
mistaken) edge case of the standards with (I think) little practical
use, than to make real applications slow in order to make this work. If
reviewers would rather keep to the letter of the standard, see #109303
which implements the alternative solution. I'm open to either path
forward.

Fixes #101907

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[flang] Reformat with latest clang-format and .clang-format

Original-commit: flang-compiler/f18@9fe84f45d7fd685051004678d6b5775dcc4c6f8f
Reviewed-on: https://github.com/flang-compiler/f18/pull/1094

[flang] Add semantic checks C8104, C8105. Add tests for C8103, C8104, C8105

Original-commit: flang-compiler/f18@8134fc477e843d9bf525eeb552ba1d234140e7a7
Reviewed-on: https://github.com/flang-compile

[flang] Add semantic checks C8104, C8105. Add tests for C8103, C8104, C8105

Original-commit: flang-compiler/f18@8134fc477e843d9bf525eeb552ba1d234140e7a7
Reviewed-on: https://github.com/flang-compiler/f18/pull/1037

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