[Attributes] Clean up handling of UB implying attributes (NFC)

[Attributes] Clean up handling of UB implying attributes (NFC)

Rather than adding methods for dropping these attributes in
various places, add a function that returns an AttrBuilder with
these attributes, which can then be used with existing methods
for dropping attributes. This is with an eye on D104641, which
also needs to drop them from returns, not just parameters.

Also be more explicit about the semantics of the method in the
documentation. Refer to UB rather than Undef, which is what this
is actually about.