Basic checking for match_first blocks (#7481)

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.

toolchain/check/scope_stack.cpp

@@ -128,6 +128,11 @@ auto ScopeStack::PushForFunctionBody(SemIR::InstId scope_inst_id) -> void {
   destroy_id_stack_.PushArray();
 }
 
+auto ScopeStack::PushForMatchFirstBlock(SemIR::InstId scope_inst_id) -> void {
+  Push(scope_inst_id, SemIR::NameScopeId::None, SemIR::SpecificId::None,
+       /*lexical_lookup_has_load_error=*/false);
+}
+
 auto ScopeStack::Pop(bool check_unused) -> void {
   auto scope = scope_stack_.pop_back_val();