[RFC PATCH] avoid applying attributes to explicit specializations (PR 83871)

[Martin Sebor]

To resolve the underlying root cause of the P1 bug c++/83503
- bogus -Wattributes for const and pure on function template
specialization, that we discussed last week, I've taken a stab
at making the change to avoid applying primary template's
attributes to its explicit specializations.  (The bug tracking
the underlying root cause is 83871 - wrong code for attribute
const and pure on distinct template specializations).

The attached patch is what I have so far.  It's incomplete
and not all the tests pass for a couple of reasons:

1) it only handles function templates (not class templates),
    and I have no tests for those yet,
2) it isn't effective for the nonnull and returns_nonnull
    attributes because they are treated as type attributes,
3) the change to deal with attributes on function arguments
    may be unnecessary (I couldn't come up with any that would
    be propagated from the primary -- are there some?).

Before I proceed with it I first want to make sure that it should
be fixed for GCC 8, that duplicate_decls is the right place for
these changes, and that (2) should be fixed by treating those
and other such attributes by applying them to function decls.

We've talked about (2) in the past (bug 71463) but this seems
like an opportunity to revisit it and (hopefully) make a change
to treat these the same as all other function attributes rather
than type attributes.  Besides fixing the wrong code bugs and
false positives it will also make them more intuitive to use.
There have been a number of bug reports from users confused by
the -Wignored-attributes warnings caused by this unusual handling.

Thanks
Martin
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