Allow impl redecl in match_first after a definition (#7491)

Previously the last decl had to be the definition. Now we allow a
declaration after a definition, so that the user can write a match_first
block last, and put (re-)declarations of impls in it, after the
definitions have already been written elsewhere.

We track the location of the decl that was associated with a match_first
block so that we can correctly point to it in diagnostics when an impl
is written twice in match_first blocks. Since impls may not be
redeclared across an import boundary, we will never have a `SemIR::Impl`
with a match_first from a different file in a redeclaration, so we don't
need to import the location of a previous decl that was in a match_first
for diagnostics. As such we just store a LocId on the `SemIR::Impl`
struct.

toolchain/sem_ir/formatter.cpp

@@ -518,6 +518,15 @@ auto Formatter::FormatImpl(ImplId id, const Impl& impl_info) -> void {
       FormatNameScope(impl_info.scope_id);
     }
 
+    if (impl_info.match_first_id.has_value()) {
+      Indent();
+      out() << "match_first = position " << impl_info.match_first_position;
+      if (impl_info.match_first_is_final) {
+        out() << ", final";
+      }
+      out() << "\n";
+    }
+
     Indent();
     out() << "witness = ";
     FormatArg(impl_info.witness_id);