Restructure handling of expressions in patterns (#7445)

Instead of maintaining a stack of pending subpatterns which might or
might not contain expressions, we mark non-nesting regions during
pattern handling that might contain an expression. The implementation
remains largely the same; the difference is that callers are expected to
end a pending expression region as soon as possible, rather than wait
for the end of the subpattern. This makes it possible to emit
non-pattern insts during pattern handling, without the risk that they
will get caught in a pending expression region further up the stack.

toolchain/check/handle_loop_statement.cpp

@@ -117,14 +117,14 @@ auto HandleParseNode(Context& context, Parse::ForHeaderStartId node_id)
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Let>();
   context.pattern_block_stack().Push();
   context.full_pattern_stack().PushNameBindingDecl();
-  BeginSubpattern(context);
+  BeginExprRegionForPattern(context);
 
   context.node_stack().Push(node_id);
   return true;
 }
 
 auto HandleParseNode(Context& context, Parse::ForInId node_id) -> bool {
-  EndSubpattern(context, context.node_stack());
+  EndExprRegionForPattern(context, context.node_stack());
   auto pattern_block_id = context.pattern_block_stack().Pop();
   AddInst<SemIR::NameBindingDecl>(context, node_id,
                                   {.pattern_block_id = pattern_block_id});