errors: simplify referring to fluent attributes

To render the message of a Fluent attribute, the identifier of the
Fluent message must be known. `DiagnosticMessage::FluentIdentifier`
contains both the message's identifier and optionally the identifier of
an attribute. Generated constants for each attribute would therefore
need to be named uniquely (amongst all error messages) or be able to
refer to only the attribute identifier which will be combined with a
message identifier later. In this commit, the latter strategy is
implemented as part of the `Diagnostic` type's functions for adding
subdiagnostics of various kinds.

Signed-off-by: David Wood <david.wood@huawei.com>

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