In `-fbounds-safety`, bounds annotations are considered type attributes
rather than declaration attributes. Constructing them as type attributes
allows us to extend the attribute to apply nested pointers, which is
essential to annotate functions that involve out parameters: `void
foo(int *__counted_by(*out_count) *out_buf, int *out_count)`.
We introduce a new sugar type to support bounds annotated types,
`CountAttributedType`. In order to maintain extra data (the bounds
expression and the dependent declaration information) that is not
trackable in `AttributedType` we create a new type dedicate to this
functionality.
This patch also extends the parsing logic to parse the `counted_by`
argument as an expression, which will allow us to extend the model to
support arguments beyond an identifier, e.g., `__counted_by(n + m)` in
the future as specified by `-fbounds-safety`.
This also adjusts `__bdos` and array-bounds sanitizer code that already
uses `CountedByAttr` to check `CountAttributedType` instead to get the
field referred to by the attribute.
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