When we check an `impl` decl, find the containing `match_first` block,
if any, and store a connection to it in the `SemIR::Impl` structure,
along with the impl's position in that `match_first` block so that we
can sort/prioritize the `SemIR::Impl`s later.
Also, update the `is_final` flag if the `match_first` block is modified
as `final`. But ensure we diagnose trying to put a `final impl`, or a
redeclaration of one, in a `match_first` block. Also diagnose if an impl
is attached to a `match_first` block more than once - either in two
different blocks or in the same block at different positions.
Putting the `match_first` connection on the `SemIR::Impl` structure
means we have to import it, so implement import and add a smoke test for
that, which ensures nothing explodes.
Drop the `scope_stack` entry for the `match_first` block, as it was not
needed. Once we started tracking the `match_first` size on the `Context`
class, it became more straightforward to just store the `match_first`
decl `InstId` in the same place. The `match_first` block is not supposed
to act like a different scope for the purpose of redecls anyhow, so it's
a bit simpler this way.
It must be in a namespace, function, or class. In particular, it can't
be in another match_first, doing so immediately associates any impl
inside with two different match_first scopes, but we only want them
associated with one for prioritization.
Push a scope_stack entry for match_first blocks, and handle impls being
inside those scope entries. An impl should not use the `match_first`
block as its "enclosing scope" for the purpose of deciding if the impl
is a redecl of another impl. We should look through it to the class or
namespace the impl (and match_first) are located inside.
We don't yet store the relationship between the impl and its match_first
block, nor then can we use it in impl lookup for prioritization.
`match_first` is a declaration followed by a curly-brace block of
declarations.
The check phase will ensure only impl decls (or defns) are inside the
block.
Dana Jansens