Parse vars in classes the same as vars in other locations (#7188)

Class vars are still restricted to simple `name: type` bindings, not
full patterns. This is now handled in the check phase instead of during
parsing.

This is in preparation for supporting `static var`.

toolchain/check/handle_pattern_list.cpp

@@ -7,6 +7,7 @@
 #include "toolchain/check/inst.h"
 #include "toolchain/check/pattern.h"
 #include "toolchain/check/type.h"
+#include "toolchain/diagnostics/emitter.h"
 
 namespace Carbon::Check {
 
@@ -95,6 +96,15 @@ auto HandleParseNode(Context& context, Parse::TuplePatternId node_id) -> bool {
   llvm::SmallVector<SemIR::InstId> type_inst_ids;
   type_inst_ids.reserve(inst_block.size());
   for (auto inst : inst_block) {
+    if (context.full_pattern_stack().IsCurrentKindFieldDecl()) {
+      CARBON_DIAGNOSTIC(FieldWithTuplePattern, Error,
+                        "found tuple pattern in class `var` decl");
+      context.emitter().Emit(LocIdForDiagnostics::TokenOnly(node_id),
+                             FieldWithTuplePattern);
+
+      return false;
+    }
+
     auto type_id = ExtractScrutineeType(context.sem_ir(),
                                         context.insts().Get(inst).type_id());
     type_inst_ids.push_back(context.types().GetTypeInstId(type_id));